Added tensor size warning to F.mse_loss() (#18349)
Summary:
To address the issue of broadcasting giving the wrong result in `nn.MSELoss()` as mentioned here https://github.com/pytorch/pytorch/issues/16045 . In particular, the issue often arises when computing the loss between tensors with shapes (n, 1) and (n,)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18349
Differential Revision: D14594176
Pulled By: soumith
fbshipit-source-id: f23ae68a4bf42f3554ad7678a314ba2c7532a6db
