Fix fetch_op_handle

Fix CI 

Fix some image deadlinks in fluid documentation

Merge branch 'develop' of https://github.com/PaddlePaddle/Paddle into deadlint push origin deadlinks

Add classification_cost to v2 documentation

* refine the document

* Update README.md

refine io_convert and op_convert

* change to readme

* Update README.md

* Save the base of the new test.

* Create the notest_recognize_digits_conv and notest_recognize_digits_mlp

* precommit check

* Change the function name from _network to _program.
Update several variable names to make them more consistant.
Update the Inferencer construction call.

* Fix the incorrect format.

* Add label semantic examples with new api

* Address review comments

* Address review comment

* Fix clone function of Program to avoid memory leak.

* Fix inference_optimize function of framework.py.

* Reuse inference_optimize in framework.py.

* Add comments.

* fix trainer import error on ce

* fix setup.py.in

timeline for distributed training

Add comment to explain how to run inference test

fix fluid Metric

* init

* init

* add ut

* split singleton from base class

* add singleton

* ad singleton