class_docstr: All reduce used by SyncBatchNorm
class_docstr_tok: ["All","reduce","used","by","SyncBatchNorm"]
code: def backward(ctx, grad_output):
      dist.all_reduce(grad_output, async_op=False)
      return grad_output
  
imports: import logging
  import torch
  import torch.distributed
  from torch import nn
  from torch.autograd.function import Function
  from torch.nn import functional
  from torch.nn import BatchNorm2d
  from ..utils.comm import get_world_size
label_desc: Use image node_id 2 for calling the AllReduce obj's underlying member method code with example usage: obj.backward(ctx, grad_output) and returns: grad_output