Evaluators are not included in V2 doc.

[lcy-seso]

I find evaluators cannot be found in V2 API doc. This is inconvenient and not friendly to the users.

[luotao1]

I will fix it ASAP.

[lcy-seso]

It seems that evaluators cannot be found in V2 API doc again.

[qingqing01]

The doc has contained evaluators:
http://doc.paddlepaddle.org/develop/doc_cn/api/v2/config/evaluators.html