Fix the documentation for BCEWithLogitsLoss (#17218, #16804) (#19212)
Summary:
I fixed a mistake in the explanation of `pos_weight` argument in `BCEWithLogitsLoss` and added an example.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19212
Differential Revision: D14923431
Pulled By: ezyang
fbshipit-source-id: 15696c67d56789102ac72afbe9bdd7b667eae5a0
