NVRTC compile options

[Yanqi-Chen]

There are many compile options concerning speed or memory cost.

Comment below if anyone has new ideas about the execution efficiency of CUDA codes.

Note: the default compile backend of cupy is nvrtc rather than nvcc, which is faster indeed.

[Yanqi-Chen]

For #110, replace mul and add with fma will not improve computing efficiency since NVCC will optimize it automatically when the flag -use_fast_math is set.

This is because setting -use_fast_math will also enable the flag -fmad, which enables the contraction of floating-point multiplies and adds/subtracts into floating-point multiply-add operations (FMAD, FFMA, or DFMA). (See https://docs.nvidia.com/cuda/cuda-compiler-driver-nvcc/index.html#options-for-steering-gpu-code-generation-fmad)