add the api and doc for the margin_ranking_loss and MarginRankingLoss

change api name eps to epsilon for the pair_distance 

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

add pairewise distance for the paddlepaddle api 2.0

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

* add Pad2D and Leaky_ReLU, test=develop

* update,test=develop

* change name,test=develop

* add unittest and redine docs,test=develop

* test=develop update interpolate & upsample api

* 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

* test=develop add linear interpolate operator

* 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 approximation for gelu, test=develop

* add functional conv

* add test and doc for function convs, test=develop

* update ConvTransposeOp's InferShape and error message, test=develop

* add hsigmoid, row_conv in paddle.nn(functional and layer), test=develop

* fix hyperlinks in docstring

* 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 instance_norm op for dygraph, test=develop

* add error message, test=develop

* add Conv/ConvTranspose layers in paddle.nn.layer, test=develop

* add example code in docstring, test=develop

* update unittests to work with test_runner, test=develop

* add L1Loss

* support L1Loss, test=develop

* add test, test=develop

* fix batch, test=develop

* follow comments, test=develop