* Refine the beam_search op and test.

* A basic CUDA implementation of beam_search for small batch_size.

* Implement CUDA kernel for beam_search_op.

* Use multiple CUDA threads in the same block to select the top beam.

* Update the python api of beam_search op.

* Enable extend function in CPU kernel of beam_search op.

* Unify the CUDA codes.
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* Unify the CPU kernel of beam_search op.

* Ensure the seletced items of beam_search_op's CPU kernel sorted by scores.

* Update the description of beam_search in API.spec.

* Enable the use of CUDA kernel in beam_search op.

* Exclude the beam_search's CUDA unittest when there is no CUDA gpu, and delete some debuging statements.
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* Follow comments.
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* Call the CPU kernel for beam_search op when batch_size > 4.
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* Remove the except of is_empty op in PrepareData.
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checkpoint for distributed training.

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This reverts commit 358e657f.

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This reverts commit 064512aa.

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* remove workspace_handle in conv2d_cudnn
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* remove workspace_handle
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* fix bug
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* make test_conv2d_op SERIAL
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* save memory in conv_cudnn
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* enhance thread safety
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* enhance temporary allocator
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* Add excess fraction
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* follow comments
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* fix bug and code refine
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* fix memory size check
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* rename reuse_tmp_allocation_excess_fraction
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* Enable basic MKL-DNN INT8 Conv OP
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* Modify test case
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* Clean unittest code
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* Fix test
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* Modify test
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* Enable MKL-DNN INT8 Conv with Relu Fusion OP
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* Enable INT8 Conv with residual fusion OP
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* Modify code.
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* Modify basic INT8 Conv
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* Modify Conv.
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* fix style
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* Fix style
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* Fix test
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* Modify code.
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* Fix test
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* Enable basic MKL-DNN INT8 Conv OP
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* Modify test case
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* Clean unittest code
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* Fix test
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* Modify test
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* Enable MKL-DNN INT8 Conv with Relu Fusion OP
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* Modify basic INT8 Conv
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* fix type
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* Modify test
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* Enable basic MKL-DNN INT8 Conv OP
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* Modify test case
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* Clean unittest code
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* Fix test
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* Modify test
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* Modify basic INT8 Conv
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* wip

* wip

* wip

* wip for test

* add fp16 tests test=develop

* fix cpu build test=develop

* fix test=develop

* fix py3 tests test=develop

* fix lr_scheduler dtype test=develop

* fix test=dvelop

* test fix ci compile test=develop

* fix build and merge test=develop

* fallback momentumop change to general test=develop

* make fp16 lr schedule simple test=develop

* fix ut test=develop

* fix tests test=develop

* remove fp16 learning rate cast test=develop

* refine tensor
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* refine tensor
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* fix device_context log
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2. Add unit test for new enforce feature

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