Add doc for nn.MultiMarginLoss (shape, example) (#63760)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/63747
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63760
Reviewed By: malfet
Differential Revision: D30541581
Pulled By: jbschlosser
fbshipit-source-id: 99560641e614296645eb0e51999513f57dfcfa98
