* add soft_label and axis for CrossEntropyLoss and improve performance,test=develop

* fix conflict in nn/functional/loss.py, test=develop

* upgrade comment string to raw string

* fix string in

* fix string with ' '

* revert update on comments

* upgrade only necessary

* fix sample code checker

* fix comments with '''

* refine doc of bernoulli

* fix some problems

* fix unsqueeze

* fix squeeze

* fix doc

* fix English doc for dice_loss, log_loss, unfold and NLLLoss API test=document_fix

also fix `no_grad` alias

modified sample code of CTCLoss & ctc_loss by remove disable_static() & print([.*].numpy()) & alias, test=document_fix (#28403)

* add float64 input to ctc_loss

* modified error message of  warpctc

* update repo and tag of warpctc

* add test for warpctc with float64 input

* modified warpctc.cmake to make sure build always

* resolved sample code bug of warpctc

* add core.ops in warpctc dygraph

* fix a bug of test

* fix summary rnn

* fix kl_div bug when input shape is [1] and reduction is batchmean

fix doc, use to_tensor for the loss ops

* fix bug

* add version check

* fix docs, test=document_fix

* fix formula, test=document_fix

* add binary cross entropy with logit loss

* add functional ctc_loss and CTCLoss class.

* modified docstring of ctc_loss and CTCLoss

* add cross_entropy to nn/layer and nn/functional, test=develop

* use functional/cross_entropy in layer/CrossEntropy

* use functional/cross_entropy in layer/CrossEntropy, test=develop

Fix the API of bce loss and add functional API binary_cross_entropy 

add nn.functional.normalize API for API2.0, fix L1Loss API typo, test=develop

* upate the target to label, test=develop

* Update the code for the margin_ranking_loss, test=develop

* add Class KLDivLoss and function kl_div

add the api and doc for the margin_ranking_loss and MarginRankingLoss

* add nll loss API, update demo code of the comment

fix paddle.nn.loss.L1Loss OP, add paddle.nn.functional.l1_loss OP for API2.0

* add alias in paddle.nn and paddle.tensor test=develop

* add alias in paddle.nn and paddle.tensor dir test=develop

* fix same conflict manually test=develop

* update fc and dygraph alias test=develop

* fix initalizer.py typo test=develop

* fix bug of cross_entropy_loss,test=develop
* fix log_softmax and some comment,test=develop

* define fluid alias in paddle.tensor and paddle.nn dir test=develop

* from layer import loss in paddle.nn dir test=develop

* Indent MSELoss example docs

* Point out input tensors should be of same shape

test=develop

* Document `MSELoss` input and return parameters

test=develop,test=document_fix

* update docs, test=develop

* polish eng docs, test=develop

* Add `paddle.nn.loss.MSELoss`

test=develop

* Move to `nn/layer/loss.py`

test=develop

* Add test

test=develop

* Fix dygraph

test=develop

* Increase numel in test

test=develop

* Add test for input with more dimensions

test=develop

* add nll_loss op test=develop

* add cross_entropy_loss,test=develop
* fix some commnet,test=develop

* add bce_loss

* fix mistake

* replace paddle_enforce,test=develop

* fix,test=develop

* update,test=develop

* remove duplicate,test=develop

* update,test=develop

* update error,test=develop

* update,test=develop

* fix unittest, test=develop

* update, test=develop

* add L1Loss

* support L1Loss, test=develop

* add test, test=develop

* fix batch, test=develop

* follow comments, test=develop

* fix test for PY3 test=develop

* reduce file changes test=develop