ignore im2col if not necessary in conv 1 * 1

Refine compute code in operators

Install GCC 4.8 in Dockerfile so to work with Eigen GPU code

Add ClipLayer

* export Backward to python

add RowL2NormLayer

Return Reference Instead Pointer to GetEigenDevice

add wget in docker production image

enable operator gpu unittest

Fix pre-commit

cpu/gpu mean op and its unit test

add check in OperatorContext Input/Output

Add Backtrace for enforce

Feature/backward

* add start_record interface

* call master client in reader

* update

* add demo code in comments

* update comments

* delete unittest for recordio reader

Slice Layer