Add argmin and argmax op

add FLAGS_use_mkldnn to global control use_mkldnn

* Remove unused arg_max/min_op.h
* Remove reference parameter. Use pointer insteaded.
* undef macro
* Always set OutT as int64_t.

Simplize API Reference Documentation

fix deadlink in api_doc_std_*.md

* Expose maxout API.

* Fix code style.

fix distribute_transpiler

New api about checkpoint and models

polish sparse update logic

Fix bad code in c plus and python

* Use for_test=True in the Fluid Trainer to clone the test program

* fix typo

* Should do the samething to the inferencer

fix a small compile error on Mac

Update crop op