Multinomial alias sampling

[xpai]

Alias sampling method to speedup multinomial sampling. The alias method treats multinomial sampling as a combination of uniform sampling and bernoulli sampling. It achieves significant acceleration when repeatedly sampling from the save multinomial distribution.

Reference:

• torch.multinomial
• https://github.com/HobbitLong/CMC/blob/master/NCE/alias_multinomial.py
• pytorch-categorical: https://github.com/enewe101/pytorch-categorical/blob/master/pytorch_categorical/categorical.py
• https://www.freesion.com/article/9414383111/