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

Remove cuptiFinalize.

* Add mean_iou op.

* Add unitest for mean iou op.

* Add optional collections of confusion matrix and mean_iou.

* Fix cuda kernel.

* Refine code.
1. Merge computing in GPU to two kernel.
2. Use wrong array and correct array instead of confusion matrix.

* Add python api and fix cuda kernel.

* Fix comments.

* Small fix.

* Small fix.

In cupti samples, only cuptiFlush is used.
I can't find any places calling cuptiFinalize and
this API can error out as not_implemented in some
cuda installation.

* fix #11416

* make sgd check tape has been backwarded_

* add error message

* change dynamic to tape

* update readme link