# from https://github.com/chongruo/detectron2-ResNeSt/blob/resnest/detectron2/modeling/backbone/resnet.py
# Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved

import numpy as np
import fvcore.nn.weight_init as weight_init
import torch
import torch.nn.functional as F
from torch import nn

from detectron2.layers import (
    Conv2d,
    DeformConv,
    FrozenBatchNorm2d,
    ModulatedDeformConv,
    ShapeSpec,
    get_norm,
)

from detectron2.modeling.backbone import Backbone
from detectron2.modeling.backbone.build import BACKBONE_REGISTRY
from detectron2.modeling.backbone.resnet import ResNetBlockBase
from detectron2.modeling.backbone.fpn import FPN, LastLevelMaxPool, LastLevelP6P7

__all__ = [
    "ResNetBlockBase",
    "BasicBlock",
    "BottleneckBlock",
    "DeformBottleneckBlock",
    "BasicStem",
    "ResNet",
    "make_stage",
    "build_resnet_backbone",
]


class ResNetBlockBase(nn.Module):
    def __init__(self, in_channels, out_channels, stride):
        """
        The `__init__` method of any subclass should also contain these arguments.
        Args:
            in_channels (int):
            out_channels (int):
            stride (int):
        """
        super().__init__()
        self.in_channels = in_channels
        self.out_channels = out_channels
        self.stride = stride

    def freeze(self):
        for p in self.parameters():
            p.requires_grad = False
        FrozenBatchNorm2d.convert_frozen_batchnorm(self)
        return self


class BasicBlock(ResNetBlockBase):
    def __init__(self, in_channels, out_channels, *, stride=1, norm="BN"):
        """
        The standard block type for ResNet18 and ResNet34.
        Args:
            in_channels (int): Number of input channels.
            out_channels (int): Number of output channels.
            stride (int): Stride for the first conv.
            norm (str or callable): A callable that takes the number of
                channels and returns a `nn.Module`, or a pre-defined string
                (one of {"FrozenBN", "BN", "GN"}).
        """
        super().__init__(in_channels, out_channels, stride)

        if in_channels != out_channels:
            self.shortcut = Conv2d(
                in_channels,
                out_channels,
                kernel_size=1,
                stride=stride,
                bias=False,
                norm=get_norm(norm, out_channels),
            )
        else:
            self.shortcut = None

        self.conv1 = Conv2d(
            in_channels,
            out_channels,
            kernel_size=3,
            stride=stride,
            padding=1,
            bias=False,
            norm=get_norm(norm, out_channels),
        )

        self.conv2 = Conv2d(
            out_channels,
            out_channels,
            kernel_size=3,
            stride=1,
            padding=1,
            bias=False,
            norm=get_norm(norm, out_channels),
        )

        for layer in [self.conv1, self.conv2, self.shortcut]:
            if layer is not None:  # shortcut can be None
                weight_init.c2_msra_fill(layer)

    def forward(self, x):
        out = self.conv1(x)
        out = F.relu_(out)
        out = self.conv2(out)

        if self.shortcut is not None:
            shortcut = self.shortcut(x)
        else:
            shortcut = x

        out += shortcut
        out = F.relu_(out)
        return out


class BottleneckBlock(ResNetBlockBase):
    def __init__(
        self,
        in_channels,
        out_channels,
        *,
        bottleneck_channels,
        stride=1,
        num_groups=1,
        norm="BN",
        stride_in_1x1=False,
        dilation=1,
        avd=False,
        avg_down=False,
        radix=2,
        bottleneck_width=64,
    ):
        """
        Args:
            norm (str or callable): a callable that takes the number of
                channels and return a `nn.Module`, or a pre-defined string
                (one of {"FrozenBN", "BN", "GN"}).
            stride_in_1x1 (bool): when stride==2, whether to put stride in the
                first 1x1 convolution or the bottleneck 3x3 convolution.
        """
        super().__init__(in_channels, out_channels, stride)

        self.avd = avd and (stride>1)
        self.avg_down = avg_down
        self.radix = radix

        cardinality = num_groups
        group_width = int(bottleneck_channels * (bottleneck_width / 64.)) * cardinality 

        if in_channels != out_channels:
            if self.avg_down:
                self.shortcut_avgpool = nn.AvgPool2d(kernel_size=stride, stride=stride, 
                                                     ceil_mode=True, count_include_pad=False)
                self.shortcut = Conv2d(
                    in_channels,
                    out_channels,
                    kernel_size=1,
                    stride=1,
                    bias=False,
                    norm=get_norm(norm, out_channels),
                )
            else:
                self.shortcut = Conv2d(
                    in_channels,
                    out_channels,
                    kernel_size=1,
                    stride=stride,
                    bias=False,
                    norm=get_norm(norm, out_channels),
                )
        else:
            self.shortcut = None

        # The original MSRA ResNet models have stride in the first 1x1 conv
        # The subsequent fb.torch.resnet and Caffe2 ResNe[X]t implementations have
        # stride in the 3x3 conv
        stride_1x1, stride_3x3 = (stride, 1) if stride_in_1x1 else (1, stride)

        self.conv1 = Conv2d(
            in_channels,
            group_width,
            kernel_size=1,
            stride=stride_1x1,
            bias=False,
            norm=get_norm(norm, group_width),
        )

        if self.radix>1:
            from .splat import SplAtConv2d
            self.conv2 = SplAtConv2d(
                            group_width, group_width, kernel_size=3, 
                            stride = 1 if self.avd else stride_3x3,
                            padding=dilation, dilation=dilation, 
                            groups=cardinality, bias=False,
                            radix=self.radix, 
                            norm=norm,
                         )
        else:
            self.conv2 = Conv2d(
                group_width,
                group_width,
                kernel_size=3,
                stride=1 if self.avd else stride_3x3,
                padding=1 * dilation,
                bias=False,
                groups=num_groups,
                dilation=dilation,
                norm=get_norm(norm, group_width),
            )

        if self.avd:
            self.avd_layer = nn.AvgPool2d(3, stride, padding=1)

        self.conv3 = Conv2d(
            group_width,
            out_channels,
            kernel_size=1,
            bias=False,
            norm=get_norm(norm, out_channels),
        )

        if self.radix>1:
            for layer in [self.conv1, self.conv3, self.shortcut]:
                if layer is not None:  # shortcut can be None
                    weight_init.c2_msra_fill(layer)
        else:
            for layer in [self.conv1, self.conv2, self.conv3, self.shortcut]:
                if layer is not None:  # shortcut can be None
                    weight_init.c2_msra_fill(layer)

        # Zero-initialize the last normalization in each residual branch,
        # so that at the beginning, the residual branch starts with zeros,
        # and each residual block behaves like an identity.
        # See Sec 5.1 in "Accurate, Large Minibatch SGD: Training ImageNet in 1 Hour":
        # "For BN layers, the learnable scaling coefficient γ is initialized
        # to be 1, except for each residual block's last BN
        # where γ is initialized to be 0."

        # nn.init.constant_(self.conv3.norm.weight, 0)
        # TODO this somehow hurts performance when training GN models from scratch.
        # Add it as an option when we need to use this code to train a backbone.

    def forward(self, x):
        out = self.conv1(x)
        out = F.relu_(out)

        if self.radix>1:
            out = self.conv2(out)
        else:
            out = self.conv2(out)
            out = F.relu_(out)

        if self.avd:
            out = self.avd_layer(out)

        out = self.conv3(out)

        if self.shortcut is not None:
            if self.avg_down:
                x = self.shortcut_avgpool(x) 
            shortcut = self.shortcut(x)
        else:
            shortcut = x

        out += shortcut
        out = F.relu_(out)
        return out


class DeformBottleneckBlock(ResNetBlockBase):
    def __init__(
        self,
        in_channels,
        out_channels,
        *,
        bottleneck_channels,
        stride=1,
        num_groups=1,
        norm="BN",
        stride_in_1x1=False,
        dilation=1,
        deform_modulated=False,
        deform_num_groups=1,
        avd=False,
        avg_down=False,
        radix=2,
        bottleneck_width=64,
    ):
        """
        Similar to :class:`BottleneckBlock`, but with deformable conv in the 3x3 convolution.
        """
        super().__init__(in_channels, out_channels, stride)
        self.deform_modulated = deform_modulated
        self.avd = avd and (stride>1)
        self.avg_down = avg_down
        self.radix = radix

        cardinality = num_groups
        group_width = int(bottleneck_channels * (bottleneck_width / 64.)) * cardinality 

        if in_channels != out_channels:
            if self.avg_down:
                self.shortcut_avgpool = nn.AvgPool2d(
                    kernel_size=stride, stride=stride, 
                    ceil_mode=True, count_include_pad=False)
                self.shortcut = Conv2d(
                    in_channels,
                    out_channels,
                    kernel_size=1,
                    stride=1,
                    bias=False,
                    norm=get_norm(norm, out_channels),
                )
            else:
                self.shortcut = Conv2d(
                    in_channels,
                    out_channels,
                    kernel_size=1,
                    stride=stride,
                    bias=False,
                    norm=get_norm(norm, out_channels),
                )
        else:
            self.shortcut = None

        stride_1x1, stride_3x3 = (stride, 1) if stride_in_1x1 else (1, stride)

        self.conv1 = Conv2d(
            in_channels,
            group_width,
            kernel_size=1,
            stride=stride_1x1,
            bias=False,
            norm=get_norm(norm, group_width),
        )

        if deform_modulated:
            deform_conv_op = ModulatedDeformConv
            # offset channels are 2 or 3 (if with modulated) * kernel_size * kernel_size
            offset_channels = 27
        else:
            deform_conv_op = DeformConv
            offset_channels = 18

        self.conv2_offset = Conv2d(
            bottleneck_channels,
            offset_channels * deform_num_groups,
            kernel_size=3,
            stride=1 if self.avd else stride_3x3,
            padding=1 * dilation,
            dilation=dilation,
            groups=deform_num_groups,
        )

        if self.radix>1:
            from .splat import SplAtConv2d_dcn
            self.conv2 = SplAtConv2d_dcn(
                            group_width, group_width, kernel_size=3, 
                            stride = 1 if self.avd else stride_3x3,
                            padding=dilation, dilation=dilation, 
                            groups=cardinality, bias=False,
                            radix=self.radix, 
                            norm=norm,
                            deform_conv_op=deform_conv_op,
                            deformable_groups=deform_num_groups,
                            deform_modulated=deform_modulated,

                         )
        else:
            self.conv2 = deform_conv_op(
                bottleneck_channels,
                bottleneck_channels,
                kernel_size=3,
                stride=1 if self.avd else stride_3x3,
                padding=1 * dilation,
                bias=False,
                groups=num_groups,
                dilation=dilation,
                deformable_groups=deform_num_groups,
                norm=get_norm(norm, bottleneck_channels),
            )

        if self.avd:
            self.avd_layer = nn.AvgPool2d(3, stride, padding=1)

        self.conv3 = Conv2d(
            group_width,
            out_channels,
            kernel_size=1,
            bias=False,
            norm=get_norm(norm, out_channels),
        )

        if self.radix>1:
            for layer in [self.conv1, self.conv3, self.shortcut]:
                if layer is not None:  # shortcut can be None
                    weight_init.c2_msra_fill(layer)
        else:
            for layer in [self.conv1, self.conv2, self.conv3, self.shortcut]:
                if layer is not None:  # shortcut can be None
                    weight_init.c2_msra_fill(layer)

        nn.init.constant_(self.conv2_offset.weight, 0)
        nn.init.constant_(self.conv2_offset.bias, 0)

    def forward(self, x):
        out = self.conv1(x)
        out = F.relu_(out)

        if self.radix>1:
            offset = self.conv2_offset(out)
            out = self.conv2(out, offset)
        else:
            if self.deform_modulated:
                offset_mask = self.conv2_offset(out)
                offset_x, offset_y, mask = torch.chunk(offset_mask, 3, dim=1)
                offset = torch.cat((offset_x, offset_y), dim=1)
                mask = mask.sigmoid()
                out = self.conv2(out, offset, mask)
            else:
                offset = self.conv2_offset(out)
                out = self.conv2(out, offset)
            out = F.relu_(out)

        if self.avd:
            out = self.avd_layer(out)

        out = self.conv3(out)

        if self.shortcut is not None:
            if self.avg_down:
                x = self.shortcut_avgpool(x) 
            shortcut = self.shortcut(x)
        else:
            shortcut = x

        out += shortcut
        out = F.relu_(out)
        return out


def make_stage(block_class, num_blocks, first_stride, **kwargs):
    """
    Create a resnet stage by creating many blocks.
    Args:
        block_class (class): a subclass of ResNetBlockBase
        num_blocks (int):
        first_stride (int): the stride of the first block. The other blocks will have stride=1.
            A `stride` argument will be passed to the block constructor.
        kwargs: other arguments passed to the block constructor.
    Returns:
        list[nn.Module]: a list of block module.
    """
    blocks = []
    for i in range(num_blocks):
        blocks.append(block_class(stride=first_stride if i == 0 else 1, **kwargs))
        kwargs["in_channels"] = kwargs["out_channels"]
    return blocks


class BasicStem(nn.Module):
    def __init__(self, in_channels=3, out_channels=64, norm="BN",
                 deep_stem=False, stem_width=32):
        """
        Args:
            norm (str or callable): a callable that takes the number of
                channels and return a `nn.Module`, or a pre-defined string
                (one of {"FrozenBN", "BN", "GN"}).
        """
        super().__init__()
        self.deep_stem = deep_stem

        if self.deep_stem:
            self.conv1_1 = Conv2d(3, stem_width, kernel_size=3, stride=2, 
                                  padding=1, bias=False,
                                  norm=get_norm(norm, stem_width),
                                 ) 
            self.conv1_2 = Conv2d(stem_width, stem_width, kernel_size=3, stride=1,
                                  padding=1, bias=False,
                                  norm=get_norm(norm, stem_width),
                                 ) 
            self.conv1_3 = Conv2d(stem_width, stem_width*2, kernel_size=3, stride=1,
                                  padding=1, bias=False,
                                  norm=get_norm(norm, stem_width*2),
                                 ) 
            for layer in [self.conv1_1, self.conv1_2, self.conv1_3]:
                if layer is not None:  
                    weight_init.c2_msra_fill(layer)
        else:
            self.conv1 = Conv2d(
                in_channels,
                out_channels,
                kernel_size=7,
                stride=2,
                padding=3,
                bias=False,
                norm=get_norm(norm, out_channels),
            )
            weight_init.c2_msra_fill(self.conv1)

    def forward(self, x):
        if self.deep_stem:
            x = self.conv1_1(x)
            x = F.relu_(x)
            x = self.conv1_2(x)
            x = F.relu_(x)
            x = self.conv1_3(x)
            x = F.relu_(x)
        else:
            x = self.conv1(x)
            x = F.relu_(x)
        x = F.max_pool2d(x, kernel_size=3, stride=2, padding=1)
        return x

    @property
    def out_channels(self):
        if self.deep_stem:
            return self.conv1_3.out_channels
        else:
            return self.conv1.out_channels

    @property
    def stride(self):
        return 4  # = stride 2 conv -> stride 2 max pool


class ResNet(Backbone):
    def __init__(self, stem, stages, num_classes=None, out_features=None):
        """
        Args:
            stem (nn.Module): a stem module
            stages (list[list[ResNetBlock]]): several (typically 4) stages,
                each contains multiple :class:`ResNetBlockBase`.
            num_classes (None or int): if None, will not perform classification.
            out_features (list[str]): name of the layers whose outputs should
                be returned in forward. Can be anything in "stem", "linear", or "res2" ...
                If None, will return the output of the last layer.
        """
        super(ResNet, self).__init__()
        self.stem = stem
        self.num_classes = num_classes

        current_stride = self.stem.stride
        self._out_feature_strides = {"stem": current_stride}
        self._out_feature_channels = {"stem": self.stem.out_channels}

        self.stages_and_names = []
        for i, blocks in enumerate(stages):
            for block in blocks:
                assert isinstance(block, ResNetBlockBase), block
                curr_channels = block.out_channels
            stage = nn.Sequential(*blocks)
            name = "res" + str(i + 2)
            self.add_module(name, stage)
            self.stages_and_names.append((stage, name))
            self._out_feature_strides[name] = current_stride = int(
                current_stride * np.prod([k.stride for k in blocks])
            )
            self._out_feature_channels[name] = blocks[-1].out_channels

        if num_classes is not None:
            self.avgpool = nn.AdaptiveAvgPool2d((1, 1))
            self.linear = nn.Linear(curr_channels, num_classes)

            # Sec 5.1 in "Accurate, Large Minibatch SGD: Training ImageNet in 1 Hour":
            # "The 1000-way fully-connected layer is initialized by
            # drawing weights from a zero-mean Gaussian with standard deviation of 0.01."
            nn.init.normal_(self.linear.weight, std=0.01)
            name = "linear"

        if out_features is None:
            out_features = [name]
        self._out_features = out_features
        assert len(self._out_features)
        children = [x[0] for x in self.named_children()]
        for out_feature in self._out_features:
            assert out_feature in children, "Available children: {}".format(", ".join(children))

    def forward(self, x):
        outputs = {}
        x = self.stem(x)
        if "stem" in self._out_features:
            outputs["stem"] = x
        for stage, name in self.stages_and_names:
            x = stage(x)
            if name in self._out_features:
                outputs[name] = x
        if self.num_classes is not None:
            x = self.avgpool(x)
            x = torch.flatten(x, 1)
            x = self.linear(x)
            if "linear" in self._out_features:
                outputs["linear"] = x
        return outputs

    def output_shape(self):
        return {
            name: ShapeSpec(
                channels=self._out_feature_channels[name], stride=self._out_feature_strides[name]
            )
            for name in self._out_features
        }


@BACKBONE_REGISTRY.register()
def build_resnest_backbone(cfg, input_shape):
    """
    Create a ResNet instance from config.
    Returns:
        ResNet: a :class:`ResNet` instance.
    """

    depth = cfg.MODEL.RESNETS.DEPTH
    stem_width = {50: 32, 101: 64, 152: 64, 200: 64, 269: 64}[depth] 
    radix = cfg.MODEL.RESNETS.RADIX 
    deep_stem = cfg.MODEL.RESNETS.DEEP_STEM or (radix > 1)

    # need registration of new blocks/stems?
    norm = cfg.MODEL.RESNETS.NORM
    stem = BasicStem(
        in_channels=input_shape.channels,
        out_channels=cfg.MODEL.RESNETS.STEM_OUT_CHANNELS,
        norm=norm,
        deep_stem=deep_stem,
        stem_width=stem_width,
    )
    freeze_at = cfg.MODEL.BACKBONE.FREEZE_AT

    if freeze_at >= 1:
        for p in stem.parameters():
            p.requires_grad = False
        stem = FrozenBatchNorm2d.convert_frozen_batchnorm(stem)

    # fmt: off
    out_features        = cfg.MODEL.RESNETS.OUT_FEATURES
    num_groups          = cfg.MODEL.RESNETS.NUM_GROUPS
    width_per_group     = cfg.MODEL.RESNETS.WIDTH_PER_GROUP
    bottleneck_channels = num_groups * width_per_group
    in_channels         = cfg.MODEL.RESNETS.STEM_OUT_CHANNELS
    out_channels        = cfg.MODEL.RESNETS.RES2_OUT_CHANNELS
    stride_in_1x1       = cfg.MODEL.RESNETS.STRIDE_IN_1X1
    res5_dilation       = cfg.MODEL.RESNETS.RES5_DILATION
    deform_on_per_stage = cfg.MODEL.RESNETS.DEFORM_ON_PER_STAGE
    deform_modulated    = cfg.MODEL.RESNETS.DEFORM_MODULATED
    deform_num_groups   = cfg.MODEL.RESNETS.DEFORM_NUM_GROUPS
    avd                 = cfg.MODEL.RESNETS.AVD or (radix > 1)
    avg_down            = cfg.MODEL.RESNETS.AVG_DOWN or (radix > 1)
    bottleneck_width    = cfg.MODEL.RESNETS.BOTTLENECK_WIDTH
    # fmt: on
    assert res5_dilation in {1, 2}, "res5_dilation cannot be {}.".format(res5_dilation)

    num_blocks_per_stage = {
        18: [2, 2, 2, 2],
        34: [3, 4, 6, 3],
        50: [3, 4, 6, 3],
        101: [3, 4, 23, 3],
        152: [3, 8, 36, 3],
        200: [3, 24, 36, 3],
        269: [3, 30, 48, 8],
    }[depth]

    if depth in [18, 34]:
        assert out_channels == 64, "Must set MODEL.RESNETS.RES2_OUT_CHANNELS = 64 for R18/R34"
        assert not any(
            deform_on_per_stage
        ), "MODEL.RESNETS.DEFORM_ON_PER_STAGE unsupported for R18/R34"
        assert res5_dilation == 1, "Must set MODEL.RESNETS.RES5_DILATION = 1 for R18/R34"
        assert num_groups == 1, "Must set MODEL.RESNETS.NUM_GROUPS = 1 for R18/R34"

    stages = []

    # Avoid creating variables without gradients
    # It consumes extra memory and may cause allreduce to fail
    out_stage_idx = [{"res2": 2, "res3": 3, "res4": 4, "res5": 5}[f] for f in out_features]
    max_stage_idx = max(out_stage_idx)
    in_channels = 2*stem_width if deep_stem else in_channels
    for idx, stage_idx in enumerate(range(2, max_stage_idx + 1)):
        dilation = res5_dilation if stage_idx == 5 else 1
        first_stride = 1 if idx == 0 or (stage_idx == 5 and dilation == 2) else 2
        stage_kargs = {
            "num_blocks": num_blocks_per_stage[idx],
            "first_stride": first_stride,
            "in_channels": in_channels,
            "out_channels": out_channels,
            "norm": norm,
            "avd": avd,
            "avg_down": avg_down,
            "radix": radix,
            "bottleneck_width": bottleneck_width,
        }
        # Use BasicBlock for R18 and R34.
        if depth in [18, 34]:
            stage_kargs["block_class"] = BasicBlock
        else:
            stage_kargs["bottleneck_channels"] = bottleneck_channels
            stage_kargs["stride_in_1x1"] = stride_in_1x1
            stage_kargs["dilation"] = dilation
            stage_kargs["num_groups"] = num_groups
            if deform_on_per_stage[idx]:
                stage_kargs["block_class"] = DeformBottleneckBlock
                stage_kargs["deform_modulated"] = deform_modulated
                stage_kargs["deform_num_groups"] = deform_num_groups
            else:
                stage_kargs["block_class"] = BottleneckBlock
        blocks = make_stage(**stage_kargs)
        in_channels = out_channels
        out_channels *= 2
        bottleneck_channels *= 2

        if freeze_at >= stage_idx:
            for block in blocks:
                block.freeze()
        stages.append(blocks)
    return ResNet(stem, stages, out_features=out_features)



@BACKBONE_REGISTRY.register()
def build_resnest_fpn_backbone(cfg, input_shape: ShapeSpec):
    """
    Args:
        cfg: a detectron2 CfgNode

    Returns:
        backbone (Backbone): backbone module, must be a subclass of :class:`Backbone`.
    """
    bottom_up = build_resnest_backbone(cfg, input_shape)
    in_features = cfg.MODEL.FPN.IN_FEATURES
    out_channels = cfg.MODEL.FPN.OUT_CHANNELS
    backbone = FPN(
        bottom_up=bottom_up,
        in_features=in_features,
        out_channels=out_channels,
        norm=cfg.MODEL.FPN.NORM,
        top_block=LastLevelMaxPool(),
        fuse_type=cfg.MODEL.FPN.FUSE_TYPE,
    )
    return backbone


@BACKBONE_REGISTRY.register()
def build_retinanet_resnest_fpn_backbone(cfg, input_shape: ShapeSpec):
    """
    Args:
        cfg: a detectron2 CfgNode

    Returns:
        backbone (Backbone): backbone module, must be a subclass of :class:`Backbone`.
    """
    bottom_up = build_resnest_backbone(cfg, input_shape)
    in_features = cfg.MODEL.FPN.IN_FEATURES
    out_channels = cfg.MODEL.FPN.OUT_CHANNELS
    in_channels_p6p7 = bottom_up.output_shape()["res5"].channels
    backbone = FPN(
        bottom_up=bottom_up,
        in_features=in_features,
        out_channels=out_channels,
        norm=cfg.MODEL.FPN.NORM,
        top_block=LastLevelP6P7(in_channels_p6p7, out_channels),
        fuse_type=cfg.MODEL.FPN.FUSE_TYPE,
    )
    return backbone

class LastLevelP6P7_P5(nn.Module):
    """
    This module is used in RetinaNet to generate extra layers, P6 and P7 from
    C5 feature.
    """

    def __init__(self, in_channels, out_channels):
        super().__init__()
        self.num_levels = 2
        self.in_feature = "p5"
        self.p6 = nn.Conv2d(in_channels, out_channels, 3, 2, 1)
        self.p7 = nn.Conv2d(out_channels, out_channels, 3, 2, 1)
        for module in [self.p6, self.p7]:
            weight_init.c2_xavier_fill(module)

    def forward(self, c5):
        p6 = self.p6(c5)
        p7 = self.p7(F.relu(p6))
        return [p6, p7]

@BACKBONE_REGISTRY.register()
def build_p67_resnest_fpn_backbone(cfg, input_shape: ShapeSpec):
    """
    Args:
        cfg: a detectron2 CfgNode

    Returns:
        backbone (Backbone): backbone module, must be a subclass of :class:`Backbone`.
    """
    bottom_up = build_resnest_backbone(cfg, input_shape)
    in_features = cfg.MODEL.FPN.IN_FEATURES
    out_channels = cfg.MODEL.FPN.OUT_CHANNELS
    backbone = FPN(
        bottom_up=bottom_up,
        in_features=in_features,
        out_channels=out_channels,
        norm=cfg.MODEL.FPN.NORM,
        top_block=LastLevelP6P7_P5(out_channels, out_channels),
        fuse_type=cfg.MODEL.FPN.FUSE_TYPE,
    )
    return backbone