class_docstr: Video 3D stem module. Provides stem operations of Conv, BN, ReLU, MaxPool
  on input data tensor for one or multiple pathways.
code: def forward(self, x):
      assert (
          len(x) == self.num_pathways
      ), "Input tensor does not contain {} pathway".format(
          self.num_pathways
      )
      for p in range(len(x)):
          stem_name = self._stem_name(p)
          x[p] = self.blocks[stem_name](x[p])
      return x
  
imports: from typing import Sequence
  import torch.nn
  from .r2plus1_util import r2plus1_unit
label_desc: Use image node_id 4 for calling the ResNeXt3DStemMultiPathway obj's underlying member method code with example usage: obj.forward(x) and returns: x