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add module cleanup
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This reverts commit 7f6d8ace.

* code style fix, test=develop

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* Make topk op support variable k.
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* Fix tensor type.
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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

Stack OP has much higher CPU cost than expected in release mode.
Caused by DebugStringEx() in base class OperatorWithKernel. Actually
this issue occur for each OP which hasn't implement it's own
GetExpectedKernelType().

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* 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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* Init slim.

* Remove distillation demo.

* Fix import errors.
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* Fix some issues.
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* Fix configs.
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* Modify API.spec.
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* Fix format.
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* Fix format.
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* Add some comments.

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