Allow converting char tensor to numpy; add [fi]info.min (#15046)
Summary:
https://github.com/pytorch/pytorch/pull/14710 with test fixed.
Also added `finfo.min` and `iinfo.min` to get castable tensors.
cc soumith
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15046
Reviewed By: soumith
Differential Revision: D13429388
Pulled By: SsnL
fbshipit-source-id: 9a08004419c83bc5ef51d03b6df3961a9f5dbf47
