class_docstr: Gather the input from model parallel region and concatenate.
  
  Args:
      input_: input matrix.
      process_group: parallel mode.
      dim: dimension
class_docstr_tok: ["Gather","the","input","from","model","parallel","region","and","concatenate",".","Args",":","input_",":","input","matrix",".","process_group",":","parallel","mode",".","dim",":","dimension"]
code: def backward(ctx, grad_output):
      return _split(grad_output, ctx.mode, ctx.dim), None, None
  
imports: from typing import List, Optional, Union
  import torch
  import torch.distributed
  from colossalai.legacy.global_variables import tensor_parallel_env
  from colossalai.legacy.nn.layer.utils import divide
  from colossalai.legacy.tensor import ColoTensorSpec, ProcessGroup
  from colossalai.tensor import ColoTensor
label_desc: Use image node_id 3 for calling the _GatherForwardSplitBackward obj's underlying member method code with example usage: obj.backward(ctx, grad_output) and returns: _split,  None,  None