test=release/1.4

* Fix imperative bugs

test=release/1.4

* Polish code

test=release/1.4

* 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

test=develop

* Reverse iterator op's input

test=develop

* Polish code

* Change the iteration direction in ingrads' map slots

test=develop

* Polish code

test=develop

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

* test=develop, fix transformer in dygraph
/

* test=develop, ptb_rnn fix op

* test=release/1.4, refine code

* test=release/1.4, fix ci failed error

test=release/1.4

* fix the bug of all_reduce_deps_pass
test=release/1.4

* 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

test=develop

test=develop

test=develop

* add slice support for dim < 0 test=develop

test=develop

test=develop

test=develop

test=develop

* 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

test=develop

test=develop

test=develop

test=develop

test=develop