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* fix selected_rows bug
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* refine cos_sim
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Dry run tests will skip `Op.Run` and just perform job scheduling. It helps to analysis dead lock in PE.

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* add fp16 backward support
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* add sum_op fp16 test

* disable test_dist_save_load
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* add check_grad for sum

* add unit test for softmax_grad fp16
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* add scale_op unit test

* add mul_grad_op unit test for fp16

* add cross_entropy_grad and eman_grad unit test for fp16
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* fix cross_entropy unit test

* add pool2d fp16 unit test

* refine conv2d fp16 unit test
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* refine activation unit test
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* fix ci
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* follow zhihong's comment, copy from https://github.com/PaddlePaddle/Paddle/pull/12796
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This reverts commit ce7d9b07.

* exhaustive search for cuDNN conv.
* Refine code and add unit testing.
* Clean code
* Fix model load in fluid/inference and unit testing in conv2d
* Follow comments.

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