Speed data reader for IMDB dataset.

Refine the activation type getting in the LSTM operator to speed.

Add "download mklml failed" into FAQ

refine pipe_reader

* "fix threadpool style"

* "remove header"

update alexnet training data

* "add data layout"

* "need kernel registry support"

* "fix data layout"

* "reorder include headers"

* "change enum to enum class"

* "fix CI"

add mkldnn fluid design doc

Use for_range to rewrite adam

* update docs

* follow comments

* fix typo

fix typo in comments of sequence_pool_op

* init kernel hint

* fix typo

* rm unused code

* add include in op_kernel.h

* restore op_kernel since it will be moved to op_kernel_type

* change force_cpu to use_cpu

* fix compilation

* remove unused place

* fix ci

Set RelWithDebInfo flags

* "fix CopyFrom parameters"

* "fix unused Place argument"

* "fixed based on comment"

Fix embedding example

Refine CUDA profiler and delete the test file.

* implement a simple threadpool

* unlock before cv.notify

* add done function

* add lock with GetAvailable function

* delete done_

* using call_once in GetInstance

* update by comment

* update comment

* enhance unit test for multi threads task

add op_kernel_type_test

rm unsed RegisterOp method in OpRegistry

fix math_function warning