* add fp16 backward support
test=develop
* add sum_op fp16 test
* disable test_dist_save_load
test=develop
* add check_grad for sum
* add unit test for softmax_grad fp16
test=develop
* add scale_op unit test
* add mul_grad_op unit test for fp16
* add cross_entropy_grad and eman_grad unit test for fp16
test=develop
* fix cross_entropy unit test
* add pool2d fp16 unit test
* refine conv2d fp16 unit test
test=develop
* refine activation unit test
test=develop
* fix ci
test=develop
* follow zhihong's comment, copy from https://github.com/PaddlePaddle/Paddle/pull/12796
test=develop