Port `mse_loss` to structured (#72294)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/72294
Reviewed By: cpuhrsch
Differential Revision: D34306555
Pulled By: ansley
fbshipit-source-id: 1c626cec2c1dfc09f462dffe0a3de74b97989f24
(cherry picked from commit d338edb477b0c035a53f845f73092bd1136f96d4)
