add autograd opBinary("+", "-", "*", "/") and fast math functions in LLVM/CUDA

• Variable.opBinary https://shigekikarita.github.io/grain/grain.autograd.Variable.html#examples
• Intrinsic math functions https://shigekikarita.github.io/grain/grain.functions.unary.unaryFunc.html
  • LLVM http://mir.dlang.io/mir_math_common.html
  • CUDA https://docs.nvidia.com/cuda/cuda-c-programming-guide/index.html#intrinsic-functions