checkpoint for distributed training.

* 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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* mem opt

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* 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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* fix mac cmake version 3.13 test=develop

* fix again test=develop

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* refactor tree2col operator with new memory mechanism test=develop

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* Modified API according to panyx0718 test=develop

* fix API change according to heavengate test=develop

* Modify API comment test=develop

* Revert the modification of while_op in #14764.
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* Remove the dependency of GRPC_DEPS.
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* Simplify the compare op for CPU.

* Use asynchronous tensor copy in reshape_op's kernel.

* Optimize while_op for test, avoiding creating variables every time.
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* Enable the cache of kernel type and kernel function.
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* Enable profiling with gperftools.

* Remove flags for testing, and fix the linking error.
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* Delete the codes of ChooseKernel.
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* Fix bug when preparing ExecutorPrepareContext for while_op.

* Fix missing depending on grpc libraries.

* Remove the redundant print.
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* Follow comments.

* Remove the codes related to prepare the ExecutorPrepareContext for while_op.
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This reverts commit 358e657f.

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