Add Gradient Operator for mean

Refine InferShape for recurrent_network_op

"add no gradient"

Add the check of cudnn version in cudnnBatchNorm

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