class_docstr: Gather tensors from all TPU workers with support for backward propagation.
code: def backward(ctx, grad_output):
      dim = ctx.dim
      all_grad_output = xm.all_reduce(xm.REDUCE_SUM, grad_output)
      return all_grad_output.select(dim, xm.get_ordinal()), None
  
imports: import contextlib
  import logging
  import os
  import pickle
  import socket
  import subprocess
  import warnings
  from itertools import chain
  import torch
  from mmf.common.registry import registry
  from torch import distributed
label_desc: Use image node_id 2 for calling the XLAGatherLayer obj's underlying member method code with example usage: obj.backward(ctx, grad_output) and returns: all_grad_output,  None