Potential bug for SearchSpace length

[roger-zhao]

tvm/python/tvm/autotvm/task/space.py

Line 839 in 08a69d4

if self._length is None:

here, when _length is None, then initiate it to current search space length, it's almost good most of times, however if user/developer get len of search space when space is empty, then a unreasonable _length 1 will be produced( the result of empty list for np.prod is 1), so it's better for adding an extra condition such as " len(self.space_map.values()) != 0" . :)

[comaniac]

Thanks for the proposal. Could you send a PR?

[roger-zhao]

@comaniac No problem!

[junrushao]

Closed via #7175