Implement conv3d with mkldnn library

Async executor bugfix: Tensor changed to LoDTensor

support loading from memory

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* add nccl2 dist tests test=develop

* fix dist_base test=develop

* fix tests test=develop

* fix test on mac test=develop

Revert "Imperative"

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Imperative

* Refine conv_fusion_op for identity activation.
* Fix unit testing.
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add prelu cuda kernel for inference.

fix the bug for mac build. python -c error. test=develop

remove jit namespace

fix bug in dist train on hs, test=develop

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Add cudnn 5 support

support customized kernel selection

Optimize hs op

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fix a const_cast and avoid using stale program.

* test dist ut fixes test=develop

* fix cmake

* for test

fix mac ci test step, test=develop

* expose square
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* fix activation
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* Add square API
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* add necessary op

* code refine

* fix API.spec
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* fix unit test
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* add unit test sparse_grad_clip
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* fix API.spec
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* remove mac test for test_gradient_clip
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* remove selectedrows_mul_tensor
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My cool stuff