content: import numpy as np
  import torch
  import torch.nn as nn
  from mmcv.cnn import ConvModule, constant_init, normal_init, xavier_init
  from ..heads import AuxHead, RebiasHead
  from ..registry import NECKS
  
  
  class Identity(nn.Module):
      """Identity mapping."""
  
      def forward(self, x):
          return x
  
  
  class DownSample(nn.Module):
      """DownSample modules.
  
      It uses convolution and maxpooling to downsample the input feature,
      and specifies downsample position to determine `pool-conv` or `conv-pool`.
  
      Args:
          in_channels (int): Channel number of input features.
          out_channels (int): Channel number of output feature.
          kernel_size (int | tuple[int]): Same as :class:`ConvModule`.
              Default: (3, 1, 1).
          stride (int | tuple[int]): Same as :class:`ConvModule`.
              Default: (1, 1, 1).
          padding (int | tuple[int]): Same as :class:`ConvModule`.
              Default: (1, 0, 0).
          groups (int): Same as :class:`ConvModule`. Default: 1.
          bias (bool | str): Same as :class:`ConvModule`. Default: False.
          conv_cfg (dict | None): Same as :class:`ConvModule`.
              Default: dict(type='Conv3d').
          norm_cfg (dict | None): Same as :class:`ConvModule`. Default: None.
          act_cfg (dict | None): Same as :class:`ConvModule`. Default: None.
          downsample_position (str): Type of downsample position. Options are
              'before' and 'after'. Default: 'after'.
          downsample_scale (int | tuple[int]): downsample scale for maxpooling.
              It will be used for kernel size and stride of maxpooling.
              Default: (1, 2, 2).
      """
  
      def __init__(self,
                   in_channels,
                   out_channels,
                   kernel_size=(3, 1, 1),
                   stride=(1, 1, 1),
                   padding=(1, 0, 0),
                   groups=1,
                   bias=False,
                   conv_cfg=dict(type='Conv3d'),
                   norm_cfg=None,
                   act_cfg=None,
                   downsample_position='after',
                   downsample_scale=(1, 2, 2)):
          super().__init__()
          self.conv = ConvModule(
              in_channels,
              out_channels,
              kernel_size,
              stride,
              padding,
              groups=groups,
              bias=bias,
              conv_cfg=conv_cfg,
              norm_cfg=norm_cfg,
              act_cfg=act_cfg)
          assert downsample_position in ['before', 'after']
          self.downsample_position = downsample_position
          self.pool = nn.MaxPool3d(
              downsample_scale, downsample_scale, (0, 0, 0), ceil_mode=True)
  
      def forward(self, x):
          if self.downsample_position == 'before':
              x = self.pool(x)
              x = self.conv(x)
          else:
              x = self.conv(x)
              x = self.pool(x)
          return x
  
  
  class LevelFusion(nn.Module):
      """Level Fusion module.
  
      This module is used to aggregate the hierarchical features dynamic in
      visual tempos and consistent in spatial semantics. The top/bottom features
      for top-down/bottom-up flow would be combined to achieve two additional
      options, namely 'Cascade Flow' or 'Parallel Flow'. While applying a
      bottom-up flow after a top-down flow will lead to the cascade flow,
      applying them simultaneously will result in the parallel flow.
  
      Args:
          in_channels (tuple[int]): Channel numbers of input features tuple.
          mid_channels (tuple[int]): Channel numbers of middle features tuple.
          out_channels (int): Channel numbers of output features.
          downsample_scales (tuple[int | tuple[int]]): downsample scales for
              each :class:`DownSample` module. Default: ((1, 1, 1), (1, 1, 1)).
      """
  
      def __init__(self,
                   in_channels,
                   mid_channels,
                   out_channels,
                   downsample_scales=((1, 1, 1), (1, 1, 1))):
          super().__init__()
          num_stages = len(in_channels)
  
          self.downsamples = nn.ModuleList()
          for i in range(num_stages):
              downsample = DownSample(
                  in_channels[i],
                  mid_channels[i],
                  kernel_size=(1, 1, 1),
                  stride=(1, 1, 1),
                  bias=False,
                  padding=(0, 0, 0),
                  groups=32,
                  norm_cfg=dict(type='BN3d', requires_grad=True),
                  act_cfg=dict(type='ReLU', inplace=True),
                  downsample_position='before',
                  downsample_scale=downsample_scales[i])
              self.downsamples.append(downsample)
  
          self.fusion_conv = ConvModule(
              sum(mid_channels),
              out_channels,
              1,
              stride=1,
              padding=0,
              bias=False,
              conv_cfg=dict(type='Conv3d'),
              norm_cfg=dict(type='BN3d', requires_grad=True),
              act_cfg=dict(type='ReLU', inplace=True))
  
      def forward(self, x):
          out = [self.downsamples[i](feature) for i, feature in enumerate(x)]
          out = torch.cat(out, 1)
          out = self.fusion_conv(out)
  
          return out
  
  
  class SpatialModulation(nn.Module):
      """Spatial Semantic Modulation.
  
      This module is used to align spatial semantics of features in the
      multi-depth pyramid. For each but the top-level feature, a stack
      of convolutions with level-specific stride are applied to it, matching
      its spatial shape and receptive field with the top one.
  
      Args:
          in_channels (tuple[int]): Channel numbers of input features tuple.
          out_channels (int): Channel numbers of output features tuple.
      """
  
      def __init__(self, in_channels, out_channels):
          super().__init__()
  
          self.spatial_modulation = nn.ModuleList()
          for channel in in_channels:
              downsample_scale = out_channels // channel
              downsample_factor = int(np.log2(downsample_scale))
              op = nn.ModuleList()
              if downsample_factor < 1:
                  op = Identity()
              else:
                  for factor in range(downsample_factor):
                      in_factor = 2**factor
                      out_factor = 2**(factor + 1)
                      op.append(
                          ConvModule(
                              channel * in_factor,
                              channel * out_factor, (1, 3, 3),
                              stride=(1, 2, 2),
                              padding=(0, 1, 1),
                              bias=False,
                              conv_cfg=dict(type='Conv3d'),
                              norm_cfg=dict(type='BN3d', requires_grad=True),
                              act_cfg=dict(type='ReLU', inplace=True)))
              self.spatial_modulation.append(op)
  
      def forward(self, x):
          out = []
          for i, _ in enumerate(x):
              if isinstance(self.spatial_modulation[i], nn.ModuleList):
                  out_ = x[i]
                  for op in self.spatial_modulation[i]:
                      out_ = op(out_)
                  out.append(out_)
              else:
                  out.append(self.spatial_modulation[i](x[i]))
          return out
  
  
  class TemporalModulation(nn.Module):
      """Temporal Rate Modulation.
  
      The module is used to equip TPN with a similar flexibility for temporal
      tempo modulation as in the input-level frame pyramid.
  
      Args:
          in_channels (int): Channel number of input features.
          out_channels (int): Channel number of output features.
          downsample_scale (int): Downsample scale for maxpooling. Default: 8.
      """
  
      def __init__(self, in_channels, out_channels, downsample_scale=8):
          super().__init__()
  
          self.conv = ConvModule(
              in_channels,
              out_channels, (3, 1, 1),
              stride=(1, 1, 1),
              padding=(1, 0, 0),
              bias=False,
              groups=32,
              conv_cfg=dict(type='Conv3d'),
              act_cfg=None)
          self.pool = nn.MaxPool3d((downsample_scale, 1, 1),
                                   (downsample_scale, 1, 1), (0, 0, 0),
                                   ceil_mode=True)
  
      def forward(self, x):
          x = self.conv(x)
          x = self.pool(x)
          return x
  
  
  @NECKS.register_module()
  class TPN(nn.Module):
      """TPN neck.
  
      This module is proposed in `Temporal Pyramid Network for Action Recognition
      <https://arxiv.org/pdf/2004.03548.pdf>`_
  
      Args:
          in_channels (tuple[int]): Channel numbers of input features tuple.
          out_channels (int): Channel number of output feature.
          spatial_modulation_cfg (dict | None): Config for spatial modulation
              layers. Required keys are `in_channels` and `out_channels`.
              Default: None.
          temporal_modulation_cfg (dict | None): Config for temporal modulation
              layers. Default: None.
          upsample_cfg (dict | None): Config for upsample layers. The keys are
              same as that in :class:``nn.Upsample``. Default: None.
          downsample_cfg (dict | None): Config for downsample layers.
              Default: None.
          level_fusion_cfg (dict | None): Config for level fusion layers.
              Required keys are 'in_channels', 'mid_channels', 'out_channels'.
              Default: None.
          aux_head_cfg (dict | None): Config for aux head layers.
              Required keys are 'out_channels'. Default: None.
          flow_type (str): Flow type to combine the features. Options are
              'cascade' and 'parallel'. Default: 'cascade'.
      """
  
      def __init__(self,
                   in_channels,
                   out_channels,
                   spatial_modulation_cfg=None,
                   temporal_modulation_cfg=None,
                   upsample_cfg=None,
                   downsample_cfg=None,
                   level_fusion_cfg=None,
                   aux_head_cfg=None,
                   rebias_head_cfg=None,
                   flow_type='cascade'):
          super().__init__()
          assert isinstance(in_channels, tuple)
          assert isinstance(out_channels, int)
          self.in_channels = in_channels
          self.out_channels = out_channels
          self.num_tpn_stages = len(in_channels)
  
          assert spatial_modulation_cfg is None or isinstance(
              spatial_modulation_cfg, dict)
          assert temporal_modulation_cfg is None or isinstance(
              temporal_modulation_cfg, dict)
          assert upsample_cfg is None or isinstance(upsample_cfg, dict)
          assert downsample_cfg is None or isinstance(downsample_cfg, dict)
          assert aux_head_cfg is None or isinstance(aux_head_cfg, dict)
          assert rebias_head_cfg is None or isinstance(rebias_head_cfg, dict)
          assert level_fusion_cfg is None or isinstance(level_fusion_cfg, dict)
  
          if flow_type not in ['cascade', 'parallel']:
              raise ValueError(
                  f"flow type in TPN should be 'cascade' or 'parallel', "
                  f'but got {flow_type} instead.')
          self.flow_type = flow_type
  
          self.temporal_modulation_ops = nn.ModuleList()
          self.upsample_ops = nn.ModuleList()
          self.downsample_ops = nn.ModuleList()
  
          self.level_fusion_1 = LevelFusion(**level_fusion_cfg)
          self.spatial_modulation = SpatialModulation(**spatial_modulation_cfg)
  
          for i in range(self.num_tpn_stages):
  
              if temporal_modulation_cfg is not None:
                  downsample_scale = temporal_modulation_cfg[
                      'downsample_scales'][i]
                  temporal_modulation = TemporalModulation(
                      in_channels[-1], out_channels, downsample_scale)
                  self.temporal_modulation_ops.append(temporal_modulation)
  
              if i < self.num_tpn_stages - 1:
                  if upsample_cfg is not None:
                      upsample = nn.Upsample(**upsample_cfg)
                      self.upsample_ops.append(upsample)
  
                  if downsample_cfg is not None:
                      downsample = DownSample(out_channels, out_channels,
                                              **downsample_cfg)
                      self.downsample_ops.append(downsample)
  
          out_dims = level_fusion_cfg['out_channels']
  
          # two pyramids
          self.level_fusion_2 = LevelFusion(**level_fusion_cfg)
  
          self.pyramid_fusion = ConvModule(
              out_dims * 2,
              2048,
              1,
              stride=1,
              padding=0,
              bias=False,
              conv_cfg=dict(type='Conv3d'),
              norm_cfg=dict(type='BN3d', requires_grad=True))
  
          if aux_head_cfg is not None:
              self.aux_head = AuxHead(self.in_channels[-2], **aux_head_cfg)
          else:
              self.aux_head = None
          if rebias_head_cfg is not None:
              self.rebias_head = RebiasHead(self.in_channels[-2], **rebias_head_cfg)
          else:
              self.rebias_head = None
          self.init_weights()
  
      # default init_weights for conv(msra) and norm in ConvModule
      def init_weights(self):
          for m in self.modules():
              if isinstance(m, nn.Conv3d):
                  xavier_init(m, distribution='uniform')
              if isinstance(m, nn.BatchNorm3d):
                  constant_init(m, 1)
  
          if self.aux_head is not None:
              self.aux_head.init_weights()
  
          if self.rebias_head is not None:
              self.rebias_head.init_weights()
          
  
      def forward(self, x, target=None):
          loss_aux = dict()
  
          # Auxiliary loss
          if self.aux_head is not None:
              loss_aux = self.aux_head(x[-2], target)
          if self.rebias_head is not None:
              loss_rebias = self.rebias_head(x[-2], target)
              loss_aux.update(loss_rebias)
  
          # Spatial Modulation
          spatial_modulation_outs = self.spatial_modulation(x)
  
          # Temporal Modulation
          temporal_modulation_outs = []
          for i, temporal_modulation in enumerate(self.temporal_modulation_ops):
              temporal_modulation_outs.append(
                  temporal_modulation(spatial_modulation_outs[i]))
  
          outs = [out.clone() for out in temporal_modulation_outs]
          if len(self.upsample_ops) != 0:
              for i in range(self.num_tpn_stages - 1, 0, -1):
                  outs[i - 1] = outs[i - 1] + self.upsample_ops[i - 1](outs[i])
  
          # Get top-down outs
          top_down_outs = self.level_fusion_1(outs)
  
          # Build bottom-up flow using downsample operation
          if self.flow_type == 'parallel':
              outs = [out.clone() for out in temporal_modulation_outs]
          if len(self.downsample_ops) != 0:
              for i in range(self.num_tpn_stages - 1):
                  outs[i + 1] = outs[i + 1] + self.downsample_ops[i](outs[i])
  
          # Get bottom-up outs
          botton_up_outs = self.level_fusion_2(outs)
  
          # fuse two pyramid outs
          outs = self.pyramid_fusion(
              torch.cat([top_down_outs, botton_up_outs], 1))
  
          return outs, loss_aux
  