1. finish lookup table CPU and GPU kernel
2. Add some cuda helper
3. Add some math funtor

Use EigenBlasGemm improve convolution computing performance in ARMv7 environment.

Set the default cuDNN installation path

"mul op gradient"

"add rowwise add backward op"

Add environment variable interface

Enable test_sgd_op

Fix sgd

fix EXTERNAL_LIBS in CMakeLists.txt

Rename `AsNoGradient` of VariableBuilder to `NotInGradient`

Remove flag use_mkldnn_wgt

Add MeanOp's Gradient Test And Fix Mean Op Gradient

Compare the gradient consistency between GPU and CPU calculations

Add memory.h for unique_ptr