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

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

* 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 paddle.nn.functional.mse_loss

* add name

* fix conflict

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

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

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