The command to pop a skip tensor.
  def forward(self, input):
      skip = yield pop('name')
      return f(input) + skip

Parameters:
      name (str) -- name of skip tensor
Returns:
      the skip tensor previously stashed by another layer under the
      same name
Return type:
      None
torch.distributed.pipeline.sync.skip.skippable.verify_skippables(module)
Verifies if the underlying skippable modules satisfy integrity.
Every skip tensor must have only one pair of stash and pop. If
   there are one or more unmatched pairs, it will raise "TypeError"
   with the detailed messages.
Here are a few failure cases. "verify_skippables()" will report
   failure for these cases:
  # Layer1 stashes "1to3".
  # Layer3 pops "1to3".

  nn.Sequential(Layer1(), Layer2())
  #               âââââ ?

  nn.Sequential(Layer2(), Layer3())
  #                   ? âââââ

  nn.Sequential(Layer1(), Layer2(), Layer3(), Layer3())
