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

Fix sparse vars usage for dist train

polish sparse update code

Refine API doc string

Add document to random crop operator

Remove warning for rst file

Do not generate doc for op_role and op_attr_name

* fix dist train error

* update by comment

fix small bug in caculating speed

Infer multi-threads API Demo and UT

* "split into multiple .ccl"

* "refine file structure"

* "refine files"

* "remove the cmakelist"

* "fix typo"

* "fix typo"

* fix ci

fix bugs in the implementation of 'HasInput' and 'HasOutput'

Fluid benchmark support recordio reader

* "init benchmark ops"

* "untrack outputs"

* "delete some usused code"

* "benchmark"

* "fix ci"

* "fix op test"

* "fix uint16 missing"

* "fix ci"

* "follow comments"

* "fix ci"

* "follow comments"

* "conficts. merge develop branch"

* repick

* "merge develop branch"