add document for bucktization (#38119)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38119
This is for (#37435).
Demo is here:
https://glaringlee.github.io/generated/torch.searchsorted.html
https://glaringlee.github.io/generated/torch.bucketize.html
Test Plan: Imported from OSS
Differential Revision: D21517392
Pulled By: glaringlee
fbshipit-source-id: b35795c7f07e9ae4c4806c528eb51fd4ca14d499
