1 metric update should be in the for loop. 2 sample_num is the shape's first element 3 preds cast should use round, astype("int32") method will cast all the elements(which is less than 1) to 0.

test=develop

* fix api format and examples

test=develop

* Update executor.py

test=develop

* Update nn.py

* Update nn.py

test=develop

* Update nn.py

test=develop

test=develop

test=develop

evaluator.py throws warnings and tell users to use DetectionMAP in metrics.py, but it is wrong in metrics.py.
So refine this API in metrics.py.

test=develop

* reimplement auc in pyton

* reimplement auc in fluid

* add auc unittest


* replace new auc in layers


* add batch Auc in Fluid

* name formated

fix AUC op
optimize it's test

* "add some api reference"

* "fix based on preview"

* "fix evaluator"

* "remove template doc"

* "add some api reference"

* "fix based on preview"

* "fix evaluator"

* "remove template doc"

* "add metrics"

* "add fluid metrics"

* "add import guards"

* "show warnings"

* "add demo"

* "fix ci"

* "add some details"

* "fix cci"

* "add demo Python"

* "add metrics"