Pass all forward op test

* Correct mul_op implementation

* Restore the origin shape after mul

* Fix mul op

* Do not touch math_function

* 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

* Init file

* Update

* Update

* Complete conv net of MNIST

* Adding incremnt op

* Fixing comment about step attribute

* Adding the interface for the momentum optimizer
* Adding a comment about accumulators

* Unify `set_feed_variable` to one method

* Move global scope to python, not in C++

* add test_fit_a_line

* Update

* fix persistable bug

* fix elementwise add bug

* set correct attr for bias op in fc layer

* set correct attr for bias op in fc layer

* Update

1. Add init_program to hold initializers
2. bug fix

* add test_fit_a_line

* fix persistable bug

* fix elementwise add bug

* fix type

* add gitignore

* Complete fit_a_line test

* revert code

* Clean up

* Revert "revert code"

This reverts commit eb1aa015.

* Refine

* Fix unit test

* Init

* unify layer names

* Update

* Add pool2d layer

* Test recognize_digits_conv

* Clean up