feat: improve sampling algorithm robustness

[yzh119]

1. add a parameter indices to sampling APIs to allow multiple samples from the same probability distribution.
2. address the abnormal case of sum(probs) < u, where our earlier implementation might return an impossible value.
3. add unittest for comparing the sampled result distribution (with a lot of trials) and original distribution.

[yzh119]

@merrymercy does the unittests look good for you?