Mistake in MSELoss documentation (#33836)
Summary:
Replaced `sum` with `mean` in [line 392](https://github.com/pytorch/pytorch/blob/master/torch/nn/modules/loss.py#L392)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/33836
Differential Revision: D20142053
Pulled By: ailzhang
fbshipit-source-id: 2bfe19944ffc5534902dd9087023e70ddf5746c3
