* enable ngraph_engine_op
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* merge develop test=develop

* avoid const_cast test=develop

* rm ngraph_operator test=develop

* Added TODO to move EnableNgraph test=develop

* Add TODO to remove const_cast test=develop

* Revert "set constant for loss"

This reverts commit 167933f6.

* Revert "remove workspace_handle"
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This reverts commit b4aca8ed.

* 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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* nce add check sample lables, test=develop

* fix github issue 15267 test=develop

* fix github issue 15267 test=develop

* monitor the GPU usage during runtime

* revert allocator_facade.cc change

* comments update test=develop

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checkpoint for distributed training.

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* Add generate_mask_labels_op to support Mask-RCNN.
* Refine sigmoid_cross_entropy to support nomalize mode.
* Fix generator_proposals_label.
* Use DeviceTemporaryAllocator in roi_pool and roi_algin.
* Remove shape check in data_feeder.

* Add check of input in sequence_expand op.
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* Correct the unittest of sequence_expand op.
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* remove workspace_handle
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* set constant for loss
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BUG: After continuing to input different data, the output cannot be aligned
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add python inference api

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* mem opt

* test=develop

* test=develop

* test=develop

* test=develop

* test=develop

* test=develop

* test=develop

* refine code  test=develop

* refine code  test=develop

* refine code  test=develop

* refine code  test=develop

* refine with cub test=develop

* fix mkldnn test && remove comments && test=develop

* polish code && test=develop

* add only_forward test && test=develop

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