Expose some activations

* "fix typo"

* "fix based on comments"

* "follow gogle style"

* "fix based on comemnts"

Tiny enhance of while_op

Doc fix and enhancement for lstm_unit python wrapper.

* "fix frigled test gradient of rnn"

* "fix based on comments"

refine pybind when *_op.cc contains several operators

* fix bug in SetAttrDescVisitor

* add comments

add norm op（cross_channel_norm）

enable MKL Packed Recurrent Layer

Dev refine backward no grad var handling

[Profiling tool]: Add the event to record the timeline. 

Add design documentation for profiling tool.

* Addign document for fluid split_lod_tensor

* Adding document for fluid merge_lod_tensor

Add doc for max_sequence_len.