v2 API doc overview

* fix nccl version

* enable nccl test

Change BN's 'mean' and 'variance' from persistable variable to Parameter

Fix compare op

add an introduction of API

Moving unique_name to python

* Also follow comments, change prefix to key

Test Parallel.Do and DynRNN

* Add reset and guard to unique_name

* framework.py enhancement

* polish

* clean up

* enforce the inputs of Operator __init__ of type Variable

* python2 assert

* reverse assert

* Add block.fwd_block_id

* fix bug in memory optimization transpiler

* Change DFS to BFS

* Add comments

Add single node tensorflow benchmark

Fix fluid dist benchmark document errors

fix a bug of sub() in layer_math.py

* Adding set_capacity method support

* Adding Python for make_channel

* Updating notest_concurrency

* Write python for make_channel method

* Write python for make_channel method

* Fix make_channel and test

* Placeholder ops for channel send, recv and close

* Adding ToTypeIndex method to var_type.h

* Add var_type.h to channel:

* Added POD_Type to the method

* Add CHANNEL to executor

* Updated get and set DataType to accomodate Channels

* Updating get and set to incorporate channels

* Adding CHANNEL as supported VarType in protobuf

* Removing unecessary import

* Fixing VarDesc to adapt to Channel as VarType

* Add channel.h to executor

* Remove innclude from channel

* Updated var_type to support Channel as  var type

* Adding get_channel to pybind

* Added ChannelHolder

* Adding make_channel as an op

* Adding ChannelHolder in channel

* Fixing typo

* Commenting out operators in concurrency

* Removing totypeid right now since we don't need it.

* Reverting python changes

* Fixing typo in framework.py

* Modify comments for ReaderHolder

* Speed up elemwise grad

* Fix bug

* Add macro for MAX_BLOCK_DIM