refine operators/math/CMakeLists.txt

[luotao1]

We should refine operators/math/CMakeLists.txt. The reason is :

• The cmake rule of CPU and GPU is separated now, which make users add two cmake rules. For examples, nv_library(pooling SRCS pooling.cc pooling.cu DEPS device_context) and cc_library(pooling SRCS pooling.cc DEPS device_context).

  Thus, we should write an automatic cmake rule and make users write op_math_library(pooling DEPS device_context) only.