code: def mean(attrs, inputs, proto_obj):
      
      concat_input = [symbol.expand_dims(op_input, axis=0) for op_input in inputs]
      concat_sym = symbol.concat(*concat_input, dim=0)
      mean_sym = symbol.mean(concat_sym, axis=0)
      return mean_sym, attrs, inputs
docs: Mean of all the input tensors.