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Otherwise, some other persistable variable (like RAW type) will not be created

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Added reading dst mem pd from lrn pd
coding style fixes

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* fix mistakes in merge_ids, test=develop

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* 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.

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* 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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1. graph and program_desc alignment
2. trt stream

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* nce add check sample lables, test=develop

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