* polish the cast op doc

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* follow comments

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* fix api.spec

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* Enable INT8 ReQuantize OP
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* Clean code
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* Add comments
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* Revert "Clean code"
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This reverts commit a7a49b8a.

* Modify requantize op test
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* fix requantize UT by moving public function to public test file.
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* Fix test fail due to file address change.
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* Change file address for requantize op.
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* refine recurrent_op
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* remove unnecessary code
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* In some case, the input may have one than one negative value.
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* fix matmul bug
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* MKLDNN: Add UT for conv_transpose_mkldnn op.
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* MKLDNN: Add fuse_bias check UT for conv_transpose_mkldnn op.
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* Revert "Add Event for TensorCopy (#15953)"

This reverts commit 7235fd66.
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* fix CI
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Add Event for TensorCopy 

* fix image_resize annotation

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* fix some typo

* Update nn.py

* Update interpolate_op.cc

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* staged.

* polish code

* polish code. test=develop

* polish code. test=develop

* api change. test=develop

* fix default value. test=develop

* fix default value. test=develop

* Rewrite is_empty op to avoid unnecessary data transform.
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* Add the implementation of InferShape and InferVarType for is_empty op.
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* Rewrite is_empty op to avoid directly inherit OperatorBase.
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* Optimize key creation of INT8 pool kernel to improve the peformance of ResNet-50 and MobileNet, especially for latency.
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* Optimize key creation of pool fp32 grad.
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