class_docstr: ResNe(X)t 3D basic stem module. Assume a single pathway.
  Performs spatiotemporal Convolution, BN, and Relu following by a
      spatiotemporal pooling.
code: def forward(self, x):
      x = self.conv(x)
      x = self.bn(x)
      x = self.relu(x)
      if self.maxpool:
          x = self.pool_layer(x)
      return x
  
imports: from typing import Sequence
  import torch.nn
  from .r2plus1_util import r2plus1_unit
label_desc: Use image node_id 3 for calling the ResNeXt3DStemSinglePathway obj's underlying member method code with example usage: obj.forward(x) and returns: x