* add pad func

* add pad

* test=develop, add pad op and apis

* restore pad2d

* test=develop, fix paddl declare

* fix pad interface

* test=develop, fix pad

* test=develop, add all pad api and cos_sim

* test=develop, remove padding default value

* test=develop, rename var to tensor

* test=develop, add more tests

* test=develop, rename tovar to totensor

* test=develop, fix init

* test=develop, add more test

* test=develop, add more tests

* add SyncBatchNorm,test=develop

* logsigmoid and LogSigmoid, test=develop

* add elu gelu relu, test=develop

* update to_variable to to_tensor, test=develop

* address review comments, test=develop

* address review comments, test=develop

* change to_variable to to_tensor in test, test=develop

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

Update the sigmoid, Sigmoid layer for the api2.0

* expose and unify the Tensor concepts to the user

* expose tensor to user

* add copy place for Tensor

* add copy place for Tensor

* add note

* add macro PADDLE_WITH_CUDA

* remove RUN_TYPE=DIST

* fix some error

* 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

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

* add base name alias; test=develop

* fix embedding bug; test=develop

* fix io error; test=develop

* add metric; test=develop

* add import palce; test=develop

* add setup; test=develop

* fix incubate reader; test=develop

* fix initializer error; test=develop

* test=develop add linear interpolate operator

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

* copy dygraph api to paddle.imperative, test=develop

* polish the code, test=develop

* polish code, test=develop

* polish code, test=develop

* move paddle.imperative.Layer to paddle.nn.Layer, 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 diag_embed op, test=develop

* add TestCase of diag_embed API

* Modified diag embed python API teastcase from dygraph to static graph, test=develop

* delete useless log and trigger ci, test=develop

* modified float16 of diag_embed, test=develop

* modified en doc of diag_embed

* trigger ci, test=develop

* add fp16 in dtype check of python API, test=develop

* modified __init__ and fix a big, test=develop

* modified a test bug of test_bicubic_interp_op and test_trilinear_interp_op, test=develop

* modified to use one kernel on cpu and cuda, 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