[caffe2] Allow LpNorm to accept empty tensor (#72660)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/72660
Sometimes it might happen when model gets an empty input.
For consistency with numpy and torch we should just return 0 without averaging or NaN with averaging.
Test Plan: Modified unittest
Differential Revision: D33782786
fbshipit-source-id: 90d8d63d685c96acc903c08c59eb39fad39e493c
(cherry picked from commit ca85779a4ea95e7500490208e359af860f613cc1)
