* Update some doc about layers' API.

* Fix format.

* Fix example bug in random_data_generator.

* Fix example bug in dropout.

* Follow comments and some small fix for some examples.

skip all tests in tests_parallel_executor_crf

Fix dist ut

Polish inline math and duplicable/optional in auto generated doc

Doc update

* fix mac build error

* enhance memory optimize transpiler to let users set to some skip opt set of variables

Polish documentation

fix deadlink in contribute_to_paddle_cn.rst

Enable more type for SplitOp and ConcatOp

Add API docs.

Add conv3d/pool3d/conv3d_trans Python API

Correct the use of the lstm layer

initial with only 1 mkl/openblas threads for each pthreads

fix compiler error in contrib/tape

Add an interface to set the number of threads for math function, and set the default value to 1 for inference.

fix a small error

* 1. Create buddy allocator in each places before NcclBcast the variables
2. Check the memory usage of ALL gpus rather than the first one

* 1. Make NCCLGroupGuard guards only the ncclBcast part, which avoid ncclGroupEnd blocking the exception throwing
2. NOTE the usage of NCCLGroupGuard

* Remove the memory usage check of gpus

* Fix code style

outvar must be create in local scope for prefetch