DOC: Clarify that CrossEntropyLoss mean is weighted (#40991)
Summary:
Closes https://github.com/pytorch/pytorch/issues/40560
This adds the equation for the weighted mean to `CrossEntropyLoss`'s docs and the `reduction` argument for `CrossEntropyLoss` and `NLLLoss` no longer describes a non-weighted mean of the outputs.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40991
Differential Revision: D22395805
Pulled By: ezyang
fbshipit-source-id: a623b6dd2aab17220fe0bf706bd9b62d6ba531fd
