* Imperative deep-first backward process (#16605)

* Fix bug of gradient interface

* shrink transformer

* Right transformer

* Change from width-first backward to deep-first backward process

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* Reverse iterator op's input

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* Polish code

* Change the iteration direction in ingrads' map slots

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* Polish code

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* test=develop, cherry-pick fix for transformer in dygraph

* test=develop, fix transformer in dygraph
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* add slice support for dim < 0 test=develop

* extend the slice function for python
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* Add fsp operator.
1 Add unitest.
2. Add python API.
3. Add layer test.

* Add quantization strategy.
1. Add API.
2. Add unitest.

* Add distillatoin strategy.

* Add unitest config file for quantization

* Fix Copyright
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* Fix setup.py

* Fix document of layers.py.
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* Fix unitest in python3.
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* Fix documents.
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* 1. refine fsp op by batched gemm
2. remove unused import
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* Fix test_dist_se_resnext.
1. disable test distillation.
2. reset framework.py
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* Enable unitest of distillation after fixing Block._clone_variable
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* Fix cdn issue.
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* Remove Desc in Forward Pass

* Refactor VarBase

* Add dbg info

* Only check type in imperative mode

* Polish code and support optimizer

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* Fix stop gradient problem in PyLayer

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* Unified ParallelExecutor and Compiler

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* Unified ParallelExecutor and Compiler

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