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test=develop

* fix some grad op desc maker
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* fix grad op desc makers
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* Fix the bug of the fast threaded executor. I

test=develop

* 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, add FC and test

* test=develop, refine code

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* add seresnet unittest test=develop

* add dropout layer test=develop

* fix ci test=develop

* fix comment test=develop

* fix comment test=develop

* fix ci test=develop

* fix ci test=develop

* fix ci

* fix module name test=develop

* run imperative serenext unit test serially test=develop

test=develop

* Add linear learning warmup method

This warmup lr can be combinated with other learning rate strategies.
For example:
            decayed_lr = fluid.layers.linear_lr_warmup(
                fluid.layers.piecewise_decay(boundaries, lr_steps),
                warmup_steps, start_lr, end_lr)

* fix cudnn batch norm accuracy test=develop

* fix cudnn batch norm accuracy test=develop

* disable failed test for later fix test=develop

test=develop

test=develop

* Add DecoupledWeightDecay

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test=develop

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* add slice support for dim < 0 test=develop

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test=develop

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test=develop

1 metric update should be in the for loop. 2 sample_num is the shape's first element 3 preds cast should use round, astype("int32") method will cast all the elements(which is less than 1) to 0.

* Add a tool to summary model's PARAMS, FLOPs in paddle/fluid/contrib.

test=develop

test=develop

test=develop

test=develop

test=develop

test=develop