HalfCauchy should ValueError if _validate_args (#50403)
Summary:
**Expected**: When I run `torch.distributions.HalfCauchy(torch.tensor(1.0), validate_args=True).log_prob(-1)`, I expect a `ValueErro` because that is the behavior of other distributions (e.g. Beta, Bernoulli).
**Actual**: No run-time error is thrown, but a `-inf` log prob is returned.
Fixes https://github.com/pytorch/pytorch/issues/50404
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This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/pytorch/pytorch/50403)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/50403
Reviewed By: mrshenli
Differential Revision: D25907131
Pulled By: neerajprad
fbshipit-source-id: ceb63537e5850809c8b32cf9db0c99043f381edf
