Multi-threaded reader in C++

Fix a bug of RecordIO

* Create select_op design document

* Fix pre-commit issues

* Update select op as per varun's comments

expose dilation to conv2d and bias/act to con2d_trans

Add float16 support to Elementwise Add op

Update index_en.rst

optimize executor.run

fix space and toctree format

Done

* Add RMSProp optimizer warpper.

* Follow comments.

pserver runs in parallel

fix #8956

repair image problem of en distributed training

1. add 1 sentence that was not translated in the original version
2. repair image showing problem

Add python wrapper for Adadelta optimizer

Enhance sequence_expand operator

Add float16 support to dropout operator

Fix bugs in c++ readers

add math_function to softmax's dep list

move distributed lookup table design to fluid/dist_train

Repair deadlink of fluid doc

repair link of "Executor"