Enhance the unit test framework to explicitly test whether the operator correctly handles gradients for multiple inputs. !3857
Created by: qingqing01
Fix #3797 (closed)
If all input gradients of forwarding operator do not need to calculate, the backward operator will be a NOP operator: https://github.com/PaddlePaddle/Paddle/blob/develop/paddle/framework/backward.cc#L75
So, there is no need to handle this case for the backward op with only one output.