Updated documentation for NLLLoss to explain what x, y and w refer to (#31488)
Summary:
Reference: https://github.com/pytorch/pytorch/issues/31385
In the current documentation for NLLLoss, it's unclear what `y` refers to in the math section of the loss description. There was an issue(https://github.com/pytorch/pytorch/issues/31295) filed earlier where there was a confusion if the loss returned for reduction=mean is right or not, perhaps because of lack in clarity of formula symbol description in the current documentation.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/31488
Differential Revision: D19181391
Pulled By: anjali411
fbshipit-source-id: 8b75f97aef93c92c26ecbce55b3faf2cd01d3e74
