Remove CUDA code from dim.h

[wangkuiyi]

It seems that caffe2::TensorShape and other tensor shape constraints are not supposed to be used in CUDA kernels. Does this mean that we can remove the CUDA code generation macros from dim.h?

[JiayiFeng]

dim serve not only as tensor shape but also as element index. In the second case, It may be used in our hand-written CUDA kernels.
Actually, it costs few to implement CUDA supporting in dim, only a few macros and function prefixes. So I tend to keep them for now. If they are turned out to be unnecessary in the future, removing them will also be an easy task.