* api2.0, Add Bilinear doc(eng,cn), test=develop

* fix some description, test=develop

* fix some description, test=develop

* fix some description, test=develop

* fix some describtion, test=develop
Co-authored-by: NChen Weihang <chenweihang@baidu.com>

* add cn doc of trace API

* add cn doc of functional ctc_loss and CTCLoss class.

* modified the format of doc.

* add en doc of ctc_loss and CTCLoss

* modified paddle.to_variable to paddle.to_tensor

* refine bceloss doc

* fix doc

* fix name

* fix api 2.0 directory migration

* add dir file and change x to input

* add doc of loss function binary_cross_entropy.

* delete static demo code.

* add 2/3d adaptive_pooling

* fix format

* fix format

* add en doc rst

* fix format

* fix doc

* fix doc

* refine example code

* unify example code

* Udpate the doc format for the paddle, test=develop

* add mse_loss_cn doc

* add index

* add index

update the margin rank loss api doc

update demo code in NLLLoss_cn.rst

Update the docs of nn.loss.L1Loss and nn.functional.l1_loss