Fix some dead links for fluid documents (cn version)

Add some comments for distribute_transpiler

Fix CPPlint issues in some operators

Fix CPPLint issues in spp_op, sum_op, topk_op, transpose_op, unpool_Op and warpctc_op

* Remove call_once.h

* "fix ci"

Add gitignores

add insert_op for block

Expose a interface to create all variables and enable the test of not creating variables every time.

* Support testing during training by ParallelExecutor.

* Add unit test.

* Improve the interface.

* Follow comments.

Merge branch 'develop' of https://github.com/PaddlePaddle/Paddle into modify_readers_to_fit_parallel_executor

Added support for dim equals 2 in activation functions

Rewrite memroy/detail

Fix debuger bugs

run prefetch prog on server

Fix seq tag model dist transpile

1. Combine 'open_files', 'multi_pass_reader' and 'threaded_reader'
together to make the new 'open_files' interface.
2. Add some docstring.
3. Simplify interface names of 'create_XXX_reader', e.g, rename
'create_double_buffer_reader' to 'double_buffer'.