[PYTHON][FFI] Skip numpy.ascontiguousarray if C_CONTIGUOUS == True

[wangxiang2713]

Hi, i just run a tensorflow model with TVM. My model has 321 inputs so i find create NDArray inputs for model is time consuming.

Code:

source_array = np.ascontiguousarray(
    source_array, dtype="uint16" if dtype == "bfloat16" else dtype
)

is used in function NDArray.copyfrom when create NDArray from Numpy. I find this code cost 0.47ms for my model, but a numpy array without operation like slice is contiguous and ascontiguousarray is not needed. In fact this code do nothing in my case, but 0.47ms is used.

So i add a type check, and i use a table DataType.NUMPY2STR to avoid str(np.dtype) as i find it time comusing, as i do in commit: #9072

[tqchen]

Thanks @wangxiang2713