fix AUC op
optimize it's test

* fix auc

* update

* update

* fix compile

* fix param name

* add doc string

* fix 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 topk Python API.

* Add unit test.

* Remove the repeated API.

* "add metrics"

* "add fluid metrics"

* "add import guards"

* "show warnings"

* "add demo"

* "fix ci"

* "add some details"

* "fix cci"

* "add demo Python"

* "add metrics"