update the margin rank loss api doc

fix unstack cn doc

update demo code in NLLLoss_cn.rst

* add pairwise distance cn docs

* fix link

* add alias

* refine format

* fix doc tree

* fix format

* add name

* change out -> output

* change eps -> epsilon

* fix name

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

* Add the max, min, maximum, minimum api for the paddle 2.0

* fix MSELoss ch doc

* support pact demo load checkpoints

* fix example code

update the api doc for the cumsum op 

* fix wrong in doc

* fix docs math style

* fix docs

* fix conv2d

* fix conv2d

update the sort api, delete the unused index output

As the title.

* Add fluid.rst

* fix cn rst

 Fix the compare ops doc for the api2.0

Fix the argsort, sort the api doc for paddle2.0

* add cholesky_cn

* add dot, inverse, test=document

* fix doc, test=document

* fix doc, test=document

fix cross op doc for API2.0

fix nce docstring for python 3