Revise error message for invalid Reduction (#22160)
Summary:
Say the user inputs reduction=False. Of course, we can't add a bool and a string, so the ValueError itself will error -which is more confusing to the user. Instead, we should use string formatting. I would use `f"{reduction} is not..."` but unsure whether we are ok with using f"" strings.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/22160
Differential Revision: D15981826
Pulled By: soumith
fbshipit-source-id: 279f34bb64a72578c36bdbabe2da83d2fa4b93d8
