TimeSformer/timesformer/models/video_model_builder.py

# Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved.

"""Video models."""

import math
import torch
import torch.nn as nn

import timesformer.utils.weight_init_helper as init_helper
from timesformer.models.batchnorm_helper import get_norm

from . import head_helper, resnet_helper, stem_helper
from .build import MODEL_REGISTRY

import math
from torch.nn import ReplicationPad3d
from torch import einsum
from einops import rearrange, reduce, repeat
import copy


import numpy as np
from timesformer.models.vit import vit_base_patch16_224

# Number of blocks for different stages given the model depth.
_MODEL_STAGE_DEPTH = {50: (3, 4, 6, 3), 101: (3, 4, 23, 3)}

# Basis of temporal kernel sizes for each of the stage.
_TEMPORAL_KERNEL_BASIS = {
    "c2d": [
        [[1]],  # conv1 temporal kernel.
        [[1]],  # res2 temporal kernel.
        [[1]],  # res3 temporal kernel.
        [[1]],  # res4 temporal kernel.
        [[1]],  # res5 temporal kernel.
    ],
    "c2d_nopool": [
        [[1]],  # conv1 temporal kernel.
        [[1]],  # res2 temporal kernel.
        [[1]],  # res3 temporal kernel.
        [[1]],  # res4 temporal kernel.
        [[1]],  # res5 temporal kernel.
    ],
    "i3d": [
        [[5]],  # conv1 temporal kernel.
        [[3]],  # res2 temporal kernel.
        [[3, 1]],  # res3 temporal kernel.
        [[3, 1]],  # res4 temporal kernel.
        [[1, 3]],  # res5 temporal kernel.
    ],
    "i3d_nopool": [
        [[5]],  # conv1 temporal kernel.
        [[3]],  # res2 temporal kernel.
        [[3, 1]],  # res3 temporal kernel.
        [[3, 1]],  # res4 temporal kernel.
        [[1, 3]],  # res5 temporal kernel.
    ],
    "slow": [
        [[1]],  # conv1 temporal kernel.
        [[1]],  # res2 temporal kernel.
        [[1]],  # res3 temporal kernel.
        [[3]],  # res4 temporal kernel.
        [[3]],  # res5 temporal kernel.
    ],
    "slowfast": [
        [[1], [5]],  # conv1 temporal kernel for slow and fast pathway.
        [[1], [3]],  # res2 temporal kernel for slow and fast pathway.
        [[1], [3]],  # res3 temporal kernel for slow and fast pathway.
        [[3], [3]],  # res4 temporal kernel for slow and fast pathway.
        [[3], [3]],  # res5 temporal kernel for slow and fast pathway.
    ],
    "x3d": [
        [[5]],  # conv1 temporal kernels.
        [[3]],  # res2 temporal kernels.
        [[3]],  # res3 temporal kernels.
        [[3]],  # res4 temporal kernels.
        [[3]],  # res5 temporal kernels.
    ],
}

_POOL1 = {
    "c2d": [[2, 1, 1]],
    "c2d_nopool": [[1, 1, 1]],
    "i3d": [[2, 1, 1]],
    "i3d_nopool": [[1, 1, 1]],
    "slow": [[1, 1, 1]],
    "slowfast": [[1, 1, 1], [1, 1, 1]],
    "x3d": [[1, 1, 1]],
}


class FuseFastToSlow(nn.Module):
    """
    Fuses the information from the Fast pathway to the Slow pathway. Given the
    tensors from Slow pathway and Fast pathway, fuse information from Fast to
    Slow, then return the fused tensors from Slow and Fast pathway in order.
    """

    def __init__(
        self,
        dim_in,
        fusion_conv_channel_ratio,
        fusion_kernel,
        alpha,
        eps=1e-5,
        bn_mmt=0.1,
        inplace_relu=True,
        norm_module=nn.BatchNorm3d,
    ):
        """
        Args:
            dim_in (int): the channel dimension of the input.
            fusion_conv_channel_ratio (int): channel ratio for the convolution
                used to fuse from Fast pathway to Slow pathway.
            fusion_kernel (int): kernel size of the convolution used to fuse
                from Fast pathway to Slow pathway.
            alpha (int): the frame rate ratio between the Fast and Slow pathway.
            eps (float): epsilon for batch norm.
            bn_mmt (float): momentum for batch norm. Noted that BN momentum in
                PyTorch = 1 - BN momentum in Caffe2.
            inplace_relu (bool): if True, calculate the relu on the original
                input without allocating new memory.
            norm_module (nn.Module): nn.Module for the normalization layer. The
                default is nn.BatchNorm3d.
        """
        super(FuseFastToSlow, self).__init__()
        self.conv_f2s = nn.Conv3d(
            dim_in,
            dim_in * fusion_conv_channel_ratio,
            kernel_size=[fusion_kernel, 1, 1],
            stride=[alpha, 1, 1],
            padding=[fusion_kernel // 2, 0, 0],
            bias=False,
        )
        self.bn = norm_module(
            num_features=dim_in * fusion_conv_channel_ratio,
            eps=eps,
            momentum=bn_mmt,
        )
        self.relu = nn.ReLU(inplace_relu)

    def forward(self, x):
        x_s = x[0]
        x_f = x[1]
        fuse = self.conv_f2s(x_f)
        fuse = self.bn(fuse)
        fuse = self.relu(fuse)
        x_s_fuse = torch.cat([x_s, fuse], 1)
        return [x_s_fuse, x_f]


@MODEL_REGISTRY.register()
class SlowFast(nn.Module):
    """
    SlowFast model builder for SlowFast network.

    Christoph Feichtenhofer, Haoqi Fan, Jitendra Malik, and Kaiming He.
    "SlowFast networks for video recognition."
    https://arxiv.org/pdf/1812.03982.pdf
    """

    def __init__(self, cfg):
        """
        The `__init__` method of any subclass should also contain these
            arguments.
        Args:
            cfg (CfgNode): model building configs, details are in the
                comments of the config file.
        """
        super(SlowFast, self).__init__()
        self.norm_module = get_norm(cfg)
        self.enable_detection = cfg.DETECTION.ENABLE
        self.num_pathways = 2

        self._construct_network(cfg)
        init_helper.init_weights(
            self, cfg.MODEL.FC_INIT_STD, cfg.RESNET.ZERO_INIT_FINAL_BN
        )


    def _construct_network(self, cfg):
        """
        Builds a SlowFast model. The first pathway is the Slow pathway and the
            second pathway is the Fast pathway.
        Args:
            cfg (CfgNode): model building configs, details are in the
                comments of the config file.
        """
        assert cfg.MODEL.ARCH in _POOL1.keys()
        pool_size = _POOL1[cfg.MODEL.ARCH]
        assert len({len(pool_size), self.num_pathways}) == 1
        assert cfg.RESNET.DEPTH in _MODEL_STAGE_DEPTH.keys()

        (d2, d3, d4, d5) = _MODEL_STAGE_DEPTH[cfg.RESNET.DEPTH]

        num_groups = cfg.RESNET.NUM_GROUPS
        width_per_group = cfg.RESNET.WIDTH_PER_GROUP
        dim_inner = num_groups * width_per_group
        out_dim_ratio = (
            cfg.SLOWFAST.BETA_INV // cfg.SLOWFAST.FUSION_CONV_CHANNEL_RATIO
        )

        temp_kernel = _TEMPORAL_KERNEL_BASIS[cfg.MODEL.ARCH]

        self.s1 = stem_helper.VideoModelStem(
            dim_in=cfg.DATA.INPUT_CHANNEL_NUM,
            dim_out=[width_per_group, width_per_group // cfg.SLOWFAST.BETA_INV],
            kernel=[temp_kernel[0][0] + [7, 7], temp_kernel[0][1] + [7, 7]],
            stride=[[1, 2, 2]] * 2,
            padding=[
                [temp_kernel[0][0][0] // 2, 3, 3],
                [temp_kernel[0][1][0] // 2, 3, 3],
            ],
            norm_module=self.norm_module,
        )
        self.s1_fuse = FuseFastToSlow(
            width_per_group // cfg.SLOWFAST.BETA_INV,
            cfg.SLOWFAST.FUSION_CONV_CHANNEL_RATIO,
            cfg.SLOWFAST.FUSION_KERNEL_SZ,
            cfg.SLOWFAST.ALPHA,
            norm_module=self.norm_module,
        )

        self.s2 = resnet_helper.ResStage(
            dim_in=[
                width_per_group + width_per_group // out_dim_ratio,
                width_per_group // cfg.SLOWFAST.BETA_INV,
            ],
            dim_out=[
                width_per_group * 4,
                width_per_group * 4 // cfg.SLOWFAST.BETA_INV,
            ],
            dim_inner=[dim_inner, dim_inner // cfg.SLOWFAST.BETA_INV],
            temp_kernel_sizes=temp_kernel[1],
            stride=cfg.RESNET.SPATIAL_STRIDES[0],
            num_blocks=[d2] * 2,
            num_groups=[num_groups] * 2,
            num_block_temp_kernel=cfg.RESNET.NUM_BLOCK_TEMP_KERNEL[0],
            nonlocal_inds=cfg.NONLOCAL.LOCATION[0],
            nonlocal_group=cfg.NONLOCAL.GROUP[0],
            nonlocal_pool=cfg.NONLOCAL.POOL[0],
            instantiation=cfg.NONLOCAL.INSTANTIATION,
            trans_func_name=cfg.RESNET.TRANS_FUNC,
            dilation=cfg.RESNET.SPATIAL_DILATIONS[0],
            norm_module=self.norm_module,
        )
        self.s2_fuse = FuseFastToSlow(
            width_per_group * 4 // cfg.SLOWFAST.BETA_INV,
            cfg.SLOWFAST.FUSION_CONV_CHANNEL_RATIO,
            cfg.SLOWFAST.FUSION_KERNEL_SZ,
            cfg.SLOWFAST.ALPHA,
            norm_module=self.norm_module,
        )

        for pathway in range(self.num_pathways):
            pool = nn.MaxPool3d(
                kernel_size=pool_size[pathway],
                stride=pool_size[pathway],
                padding=[0, 0, 0],
            )
            self.add_module("pathway{}_pool".format(pathway), pool)

        self.s3 = resnet_helper.ResStage(
            dim_in=[
                width_per_group * 4 + width_per_group * 4 // out_dim_ratio,
                width_per_group * 4 // cfg.SLOWFAST.BETA_INV,
            ],
            dim_out=[
                width_per_group * 8,
                width_per_group * 8 // cfg.SLOWFAST.BETA_INV,
            ],
            dim_inner=[dim_inner * 2, dim_inner * 2 // cfg.SLOWFAST.BETA_INV],
            temp_kernel_sizes=temp_kernel[2],
            stride=cfg.RESNET.SPATIAL_STRIDES[1],
            num_blocks=[d3] * 2,
            num_groups=[num_groups] * 2,
            num_block_temp_kernel=cfg.RESNET.NUM_BLOCK_TEMP_KERNEL[1],
            nonlocal_inds=cfg.NONLOCAL.LOCATION[1],
            nonlocal_group=cfg.NONLOCAL.GROUP[1],
            nonlocal_pool=cfg.NONLOCAL.POOL[1],
            instantiation=cfg.NONLOCAL.INSTANTIATION,
            trans_func_name=cfg.RESNET.TRANS_FUNC,
            dilation=cfg.RESNET.SPATIAL_DILATIONS[1],
            norm_module=self.norm_module,
        )
        self.s3_fuse = FuseFastToSlow(
            width_per_group * 8 // cfg.SLOWFAST.BETA_INV,
            cfg.SLOWFAST.FUSION_CONV_CHANNEL_RATIO,
            cfg.SLOWFAST.FUSION_KERNEL_SZ,
            cfg.SLOWFAST.ALPHA,
            norm_module=self.norm_module,
        )

        self.s4 = resnet_helper.ResStage(
            dim_in=[
                width_per_group * 8 + width_per_group * 8 // out_dim_ratio,
                width_per_group * 8 // cfg.SLOWFAST.BETA_INV,
            ],
            dim_out=[
                width_per_group * 16,
                width_per_group * 16 // cfg.SLOWFAST.BETA_INV,
            ],
            dim_inner=[dim_inner * 4, dim_inner * 4 // cfg.SLOWFAST.BETA_INV],
            temp_kernel_sizes=temp_kernel[3],
            stride=cfg.RESNET.SPATIAL_STRIDES[2],
            num_blocks=[d4] * 2,
            num_groups=[num_groups] * 2,
            num_block_temp_kernel=cfg.RESNET.NUM_BLOCK_TEMP_KERNEL[2],
            nonlocal_inds=cfg.NONLOCAL.LOCATION[2],
            nonlocal_group=cfg.NONLOCAL.GROUP[2],
            nonlocal_pool=cfg.NONLOCAL.POOL[2],
            instantiation=cfg.NONLOCAL.INSTANTIATION,
            trans_func_name=cfg.RESNET.TRANS_FUNC,
            dilation=cfg.RESNET.SPATIAL_DILATIONS[2],
            norm_module=self.norm_module,
        )
        self.s4_fuse = FuseFastToSlow(
            width_per_group * 16 // cfg.SLOWFAST.BETA_INV,
            cfg.SLOWFAST.FUSION_CONV_CHANNEL_RATIO,
            cfg.SLOWFAST.FUSION_KERNEL_SZ,
            cfg.SLOWFAST.ALPHA,
            norm_module=self.norm_module,
        )

        self.s5 = resnet_helper.ResStage(
            dim_in=[
                width_per_group * 16 + width_per_group * 16 // out_dim_ratio,
                width_per_group * 16 // cfg.SLOWFAST.BETA_INV,
            ],
            dim_out=[
                width_per_group * 32,
                width_per_group * 32 // cfg.SLOWFAST.BETA_INV,
            ],
            dim_inner=[dim_inner * 8, dim_inner * 8 // cfg.SLOWFAST.BETA_INV],
            temp_kernel_sizes=temp_kernel[4],
            stride=cfg.RESNET.SPATIAL_STRIDES[3],
            num_blocks=[d5] * 2,
            num_groups=[num_groups] * 2,
            num_block_temp_kernel=cfg.RESNET.NUM_BLOCK_TEMP_KERNEL[3],
            nonlocal_inds=cfg.NONLOCAL.LOCATION[3],
            nonlocal_group=cfg.NONLOCAL.GROUP[3],
            nonlocal_pool=cfg.NONLOCAL.POOL[3],
            instantiation=cfg.NONLOCAL.INSTANTIATION,
            trans_func_name=cfg.RESNET.TRANS_FUNC,
            dilation=cfg.RESNET.SPATIAL_DILATIONS[3],
            norm_module=self.norm_module,
        )

        if cfg.DETECTION.ENABLE:
            self.head = head_helper.ResNetRoIHead(
                dim_in=[
                    width_per_group * 32,
                    width_per_group * 32 // cfg.SLOWFAST.BETA_INV,
                ],
                num_classes=cfg.MODEL.NUM_CLASSES,
                pool_size=[
                    [
                        cfg.DATA.NUM_FRAMES
                        // cfg.SLOWFAST.ALPHA
                        // pool_size[0][0],
                        1,
                        1,
                    ],
                    [cfg.DATA.NUM_FRAMES // pool_size[1][0], 1, 1],
                ],
                resolution=[[cfg.DETECTION.ROI_XFORM_RESOLUTION] * 2] * 2,
                scale_factor=[cfg.DETECTION.SPATIAL_SCALE_FACTOR] * 2,
                dropout_rate=cfg.MODEL.DROPOUT_RATE,
                act_func=cfg.MODEL.HEAD_ACT,
                aligned=cfg.DETECTION.ALIGNED,
            )
        else:
            head = head_helper.ResNetBasicHead(
                dim_in=[
                    width_per_group * 32,
                    width_per_group * 32 // cfg.SLOWFAST.BETA_INV,
                ],
                num_classes=cfg.MODEL.NUM_CLASSES,
                pool_size=[None, None]
                if cfg.MULTIGRID.SHORT_CYCLE
                else [
                    [
                        cfg.DATA.NUM_FRAMES
                        // cfg.SLOWFAST.ALPHA
                        // pool_size[0][0],
                        cfg.DATA.TRAIN_CROP_SIZE // 32 // pool_size[0][1],
                        cfg.DATA.TRAIN_CROP_SIZE // 32 // pool_size[0][2],
                    ],
                    [
                        cfg.DATA.NUM_FRAMES // pool_size[1][0],
                        cfg.DATA.TRAIN_CROP_SIZE // 32 // pool_size[1][1],
                        cfg.DATA.TRAIN_CROP_SIZE // 32 // pool_size[1][2],
                    ],
                ],  # None for AdaptiveAvgPool3d((1, 1, 1))
                dropout_rate=cfg.MODEL.DROPOUT_RATE,
                act_func=cfg.MODEL.HEAD_ACT,
            )
            self.head_name = "head{}".format(cfg.TASK)
            self.add_module(self.head_name, head)

    def forward(self, x, bboxes=None):
        x = self.s1(x)
        x = self.s1_fuse(x)
        x = self.s2(x)
        x = self.s2_fuse(x)
        for pathway in range(self.num_pathways):
            pool = getattr(self, "pathway{}_pool".format(pathway))
            x[pathway] = pool(x[pathway])
        x = self.s3(x)
        x = self.s3_fuse(x)
        x = self.s4(x)
        x = self.s4_fuse(x)
        x = self.s5(x)

        head = getattr(self, self.head_name)
        if self.enable_detection:
            x = head(x, bboxes)
        else:
            x = head(x)

        return x


@MODEL_REGISTRY.register()
class ResNet(nn.Module):
    """
    ResNet model builder. It builds a ResNet like network backbone without
    lateral connection (C2D, I3D, Slow).

    Christoph Feichtenhofer, Haoqi Fan, Jitendra Malik, and Kaiming He.
    "SlowFast networks for video recognition."
    https://arxiv.org/pdf/1812.03982.pdf

    Xiaolong Wang, Ross Girshick, Abhinav Gupta, and Kaiming He.
    "Non-local neural networks."
    https://arxiv.org/pdf/1711.07971.pdf
    """

    def __init__(self, cfg):
        """
        The `__init__` method of any subclass should also contain these
            arguments.

        Args:
            cfg (CfgNode): model building configs, details are in the
                comments of the config file.
        """
        super(ResNet, self).__init__()
        self.norm_module = get_norm(cfg)
        self.enable_detection = cfg.DETECTION.ENABLE
        self.num_pathways = 1
        self._construct_network(cfg)
        init_helper.init_weights(
            self, cfg.MODEL.FC_INIT_STD, cfg.RESNET.ZERO_INIT_FINAL_BN
        )

    def _construct_network(self, cfg):
        """
        Builds a single pathway ResNet model.

        Args:
            cfg (CfgNode): model building configs, details are in the
                comments of the config file.
        """
        assert cfg.MODEL.ARCH in _POOL1.keys()
        pool_size = _POOL1[cfg.MODEL.ARCH]
        assert len({len(pool_size), self.num_pathways}) == 1
        assert cfg.RESNET.DEPTH in _MODEL_STAGE_DEPTH.keys()

        (d2, d3, d4, d5) = _MODEL_STAGE_DEPTH[cfg.RESNET.DEPTH]

        num_groups = cfg.RESNET.NUM_GROUPS
        width_per_group = cfg.RESNET.WIDTH_PER_GROUP
        dim_inner = num_groups * width_per_group

        temp_kernel = _TEMPORAL_KERNEL_BASIS[cfg.MODEL.ARCH]

        self.s1 = stem_helper.VideoModelStem(
            dim_in=cfg.DATA.INPUT_CHANNEL_NUM,
            dim_out=[width_per_group],
            kernel=[temp_kernel[0][0] + [7, 7]],
            stride=[[1, 2, 2]],
            padding=[[temp_kernel[0][0][0] // 2, 3, 3]],
            norm_module=self.norm_module,
        )

        self.s2 = resnet_helper.ResStage(
            dim_in=[width_per_group],
            dim_out=[width_per_group * 4],
            dim_inner=[dim_inner],
            temp_kernel_sizes=temp_kernel[1],
            stride=cfg.RESNET.SPATIAL_STRIDES[0],
            num_blocks=[d2],
            num_groups=[num_groups],
            num_block_temp_kernel=cfg.RESNET.NUM_BLOCK_TEMP_KERNEL[0],
            nonlocal_inds=cfg.NONLOCAL.LOCATION[0],
            nonlocal_group=cfg.NONLOCAL.GROUP[0],
            nonlocal_pool=cfg.NONLOCAL.POOL[0],
            instantiation=cfg.NONLOCAL.INSTANTIATION,
            trans_func_name=cfg.RESNET.TRANS_FUNC,
            stride_1x1=cfg.RESNET.STRIDE_1X1,
            inplace_relu=cfg.RESNET.INPLACE_RELU,
            dilation=cfg.RESNET.SPATIAL_DILATIONS[0],
            norm_module=self.norm_module,
        )

        for pathway in range(self.num_pathways):
            pool = nn.MaxPool3d(
                kernel_size=pool_size[pathway],
                stride=pool_size[pathway],
                padding=[0, 0, 0],
            )
            self.add_module("pathway{}_pool".format(pathway), pool)

        self.s3 = resnet_helper.ResStage(
            dim_in=[width_per_group * 4],
            dim_out=[width_per_group * 8],
            dim_inner=[dim_inner * 2],
            temp_kernel_sizes=temp_kernel[2],
            stride=cfg.RESNET.SPATIAL_STRIDES[1],
            num_blocks=[d3],
            num_groups=[num_groups],
            num_block_temp_kernel=cfg.RESNET.NUM_BLOCK_TEMP_KERNEL[1],
            nonlocal_inds=cfg.NONLOCAL.LOCATION[1],
            nonlocal_group=cfg.NONLOCAL.GROUP[1],
            nonlocal_pool=cfg.NONLOCAL.POOL[1],
            instantiation=cfg.NONLOCAL.INSTANTIATION,
            trans_func_name=cfg.RESNET.TRANS_FUNC,
            stride_1x1=cfg.RESNET.STRIDE_1X1,
            inplace_relu=cfg.RESNET.INPLACE_RELU,
            dilation=cfg.RESNET.SPATIAL_DILATIONS[1],
            norm_module=self.norm_module,
        )

        self.s4 = resnet_helper.ResStage(
            dim_in=[width_per_group * 8],
            dim_out=[width_per_group * 16],
            dim_inner=[dim_inner * 4],
            temp_kernel_sizes=temp_kernel[3],
            stride=cfg.RESNET.SPATIAL_STRIDES[2],
            num_blocks=[d4],
            num_groups=[num_groups],
            num_block_temp_kernel=cfg.RESNET.NUM_BLOCK_TEMP_KERNEL[2],
            nonlocal_inds=cfg.NONLOCAL.LOCATION[2],
            nonlocal_group=cfg.NONLOCAL.GROUP[2],
            nonlocal_pool=cfg.NONLOCAL.POOL[2],
            instantiation=cfg.NONLOCAL.INSTANTIATION,
            trans_func_name=cfg.RESNET.TRANS_FUNC,
            stride_1x1=cfg.RESNET.STRIDE_1X1,
            inplace_relu=cfg.RESNET.INPLACE_RELU,
            dilation=cfg.RESNET.SPATIAL_DILATIONS[2],
            norm_module=self.norm_module,
        )

        self.s5 = resnet_helper.ResStage(
            dim_in=[width_per_group * 16],
            dim_out=[width_per_group * 32],
            dim_inner=[dim_inner * 8],
            temp_kernel_sizes=temp_kernel[4],
            stride=cfg.RESNET.SPATIAL_STRIDES[3],
            num_blocks=[d5],
            num_groups=[num_groups],
            num_block_temp_kernel=cfg.RESNET.NUM_BLOCK_TEMP_KERNEL[3],
            nonlocal_inds=cfg.NONLOCAL.LOCATION[3],
            nonlocal_group=cfg.NONLOCAL.GROUP[3],
            nonlocal_pool=cfg.NONLOCAL.POOL[3],
            instantiation=cfg.NONLOCAL.INSTANTIATION,
            trans_func_name=cfg.RESNET.TRANS_FUNC,
            stride_1x1=cfg.RESNET.STRIDE_1X1,
            inplace_relu=cfg.RESNET.INPLACE_RELU,
            dilation=cfg.RESNET.SPATIAL_DILATIONS[3],
            norm_module=self.norm_module,
        )

        if self.enable_detection:
            self.head = head_helper.ResNetRoIHead(
                dim_in=[width_per_group * 32],
                num_classes=cfg.MODEL.NUM_CLASSES,
                pool_size=[[cfg.DATA.NUM_FRAMES // pool_size[0][0], 1, 1]],
                resolution=[[cfg.DETECTION.ROI_XFORM_RESOLUTION] * 2],
                scale_factor=[cfg.DETECTION.SPATIAL_SCALE_FACTOR],
                dropout_rate=cfg.MODEL.DROPOUT_RATE,
                act_func=cfg.MODEL.HEAD_ACT,
                aligned=cfg.DETECTION.ALIGNED,
            )
        else:
            head = head_helper.ResNetBasicHead(
                dim_in=[width_per_group * 32],
                num_classes=cfg.MODEL.NUM_CLASSES,
                pool_size=[None, None]
                if cfg.MULTIGRID.SHORT_CYCLE
                else [
                    [
                        cfg.DATA.NUM_FRAMES // pool_size[0][0],
                        cfg.DATA.TRAIN_CROP_SIZE // 32 // pool_size[0][1],
                        cfg.DATA.TRAIN_CROP_SIZE // 32 // pool_size[0][2],
                    ]
                ],  # None for AdaptiveAvgPool3d((1, 1, 1))
                dropout_rate=cfg.MODEL.DROPOUT_RATE,
                act_func=cfg.MODEL.HEAD_ACT,
            )
            self.head_name = "head{}".format(cfg.TASK)
            self.add_module(self.head_name, head)

    def forward(self, x, bboxes=None):
        x = self.s1(x)
        x = self.s2(x)
        for pathway in range(self.num_pathways):
            pool = getattr(self, "pathway{}_pool".format(pathway))
            x[pathway] = pool(x[pathway])
        x = self.s3(x)
        x = self.s4(x)
        x = self.s5(x)

        head = getattr(self, self.head_name)
        if self.enable_detection:
            x = head(x, bboxes)
        else:
            x = head(x)
        return x


@MODEL_REGISTRY.register()
class X3D(nn.Module):
    """
    X3D model builder. It builds a X3D network backbone, which is a ResNet.

    Christoph Feichtenhofer.
    "X3D: Expanding Architectures for Efficient Video Recognition."
    https://arxiv.org/abs/2004.04730
    """

    def __init__(self, cfg):
        """
        The `__init__` method of any subclass should also contain these
            arguments.

        Args:
            cfg (CfgNode): model building configs, details are in the
                comments of the config file.
        """
        super(X3D, self).__init__()
        self.norm_module = get_norm(cfg)
        self.enable_detection = cfg.DETECTION.ENABLE
        self.num_pathways = 1

        exp_stage = 2.0
        self.dim_c1 = cfg.X3D.DIM_C1

        self.dim_res2 = (
            self._round_width(self.dim_c1, exp_stage, divisor=8)
            if cfg.X3D.SCALE_RES2
            else self.dim_c1
        )
        self.dim_res3 = self._round_width(self.dim_res2, exp_stage, divisor=8)
        self.dim_res4 = self._round_width(self.dim_res3, exp_stage, divisor=8)
        self.dim_res5 = self._round_width(self.dim_res4, exp_stage, divisor=8)

        self.block_basis = [
            # blocks, c, stride
            [1, self.dim_res2, 2],
            [2, self.dim_res3, 2],
            [5, self.dim_res4, 2],
            [3, self.dim_res5, 2],
        ]
        self._construct_network(cfg)
        init_helper.init_weights(
            self, cfg.MODEL.FC_INIT_STD, cfg.RESNET.ZERO_INIT_FINAL_BN
        )

    def _round_width(self, width, multiplier, min_depth=8, divisor=8):
        """Round width of filters based on width multiplier."""
        if not multiplier:
            return width

        width *= multiplier
        min_depth = min_depth or divisor
        new_filters = max(
            min_depth, int(width + divisor / 2) // divisor * divisor
        )
        if new_filters < 0.9 * width:
            new_filters += divisor
        return int(new_filters)

    def _round_repeats(self, repeats, multiplier):
        """Round number of layers based on depth multiplier."""
        multiplier = multiplier
        if not multiplier:
            return repeats
        return int(math.ceil(multiplier * repeats))

    def _construct_network(self, cfg):
        """
        Builds a single pathway X3D model.

        Args:
            cfg (CfgNode): model building configs, details are in the
                comments of the config file.
        """
        assert cfg.MODEL.ARCH in _POOL1.keys()
        assert cfg.RESNET.DEPTH in _MODEL_STAGE_DEPTH.keys()

        (d2, d3, d4, d5) = _MODEL_STAGE_DEPTH[cfg.RESNET.DEPTH]

        num_groups = cfg.RESNET.NUM_GROUPS
        width_per_group = cfg.RESNET.WIDTH_PER_GROUP
        dim_inner = num_groups * width_per_group

        w_mul = cfg.X3D.WIDTH_FACTOR
        d_mul = cfg.X3D.DEPTH_FACTOR
        dim_res1 = self._round_width(self.dim_c1, w_mul)

        temp_kernel = _TEMPORAL_KERNEL_BASIS[cfg.MODEL.ARCH]

        self.s1 = stem_helper.VideoModelStem(
            dim_in=cfg.DATA.INPUT_CHANNEL_NUM,
            dim_out=[dim_res1],
            kernel=[temp_kernel[0][0] + [3, 3]],
            stride=[[1, 2, 2]],
            padding=[[temp_kernel[0][0][0] // 2, 1, 1]],
            norm_module=self.norm_module,
            stem_func_name="x3d_stem",
        )

        # blob_in = s1
        dim_in = dim_res1
        for stage, block in enumerate(self.block_basis):
            dim_out = self._round_width(block[1], w_mul)
            dim_inner = int(cfg.X3D.BOTTLENECK_FACTOR * dim_out)

            n_rep = self._round_repeats(block[0], d_mul)
            prefix = "s{}".format(
                stage + 2
            )  # start w res2 to follow convention

            s = resnet_helper.ResStage(
                dim_in=[dim_in],
                dim_out=[dim_out],
                dim_inner=[dim_inner],
                temp_kernel_sizes=temp_kernel[1],
                stride=[block[2]],
                num_blocks=[n_rep],
                num_groups=[dim_inner]
                if cfg.X3D.CHANNELWISE_3x3x3
                else [num_groups],
                num_block_temp_kernel=[n_rep],
                nonlocal_inds=cfg.NONLOCAL.LOCATION[0],
                nonlocal_group=cfg.NONLOCAL.GROUP[0],
                nonlocal_pool=cfg.NONLOCAL.POOL[0],
                instantiation=cfg.NONLOCAL.INSTANTIATION,
                trans_func_name=cfg.RESNET.TRANS_FUNC,
                stride_1x1=cfg.RESNET.STRIDE_1X1,
                norm_module=self.norm_module,
                dilation=cfg.RESNET.SPATIAL_DILATIONS[stage],
                drop_connect_rate=cfg.MODEL.DROPCONNECT_RATE
                * (stage + 2)
                / (len(self.block_basis) + 1),
            )
            dim_in = dim_out
            self.add_module(prefix, s)

        if self.enable_detection:
            NotImplementedError
        else:
            spat_sz = int(math.ceil(cfg.DATA.TRAIN_CROP_SIZE / 32.0))
            self.head = head_helper.X3DHead(
                dim_in=dim_out,
                dim_inner=dim_inner,
                dim_out=cfg.X3D.DIM_C5,
                num_classes=cfg.MODEL.NUM_CLASSES,
                pool_size=[cfg.DATA.NUM_FRAMES, spat_sz, spat_sz],
                dropout_rate=cfg.MODEL.DROPOUT_RATE,
                act_func=cfg.MODEL.HEAD_ACT,
                bn_lin5_on=cfg.X3D.BN_LIN5,
            )

    def forward(self, x, bboxes=None):
        for module in self.children():
            x = module(x)
        return x