code: def backward(ctx, grad_output):
      tensor = torch.empty(
          ctx.input_size,
          device=grad_output.device,
          dtype=grad_output.dtype,
      )
      return (None, None, None, None) + (
          _AlltoAllSingle.apply(
              ctx.group,
              tensor,
              ctx.output_split_sizes,
              ctx.input_split_sizes,
              grad_output.contiguous(),
          ),
      )
  
imports: import torch
  import torch.distributed
  from torch.autograd import Function
  from torch.distributed import group, ReduceOp
label_desc: Use image node_id 2 for calling the _AlltoAllSingle obj's underlying member method code with example usage: obj.backward(ctx, grad_output) and returns: unknown