survey on computation graph

 Enable to output LoD in fetch_op and check output LoD in the op unit test.

fix im2col interface

Add a warning to the docstring of Parameters.to_tar()

fix nccl compiler error

Add up, down, left and right padding for im2col.

deconv op implementing ...

* Add print_operators_doc.cc

* Update Escape

* Correct a bug

* Remove OpInfoMap::Iterate

* Update the print_operators_doc.cc

* Escape tab

* Use auto&

* Use auto&

* Remove trailing ,

* clang-format C++

Pass all forward op test

* Correct mul_op implementation

* Restore the origin shape after mul

* Fix mul op

* Do not touch math_function

* "add nccl enforce"

* Dev

* Update comment

* Add nccl test

* Follow comments

* init

* ensure ids in lookup table op must be a column vector

* add book4 configuration in test_layers

* debug test_book4

* add test_word2vec

* follow comments

* follow comments

* ensure ids in lookup table op must be a column vector

* follow comments

Fix a bug of dot-product attention.