Use TORCH_CHECK instead of AT_ASSERT in torch::cuda::gather() (#27456)
Summary:
The error message produced by AT_ASSERT() in gather() encouraged users to file a bug report ("please report a bug to PyTorch..."). The assertion should be a regular argument check since it can be triggered by passing tensors with different dimensionality, e.g. `torch.cuda.comm.gather([torch.rand(1, device='cuda'), torch.rand(1, 1, device='cuda')])`.
See: https://github.com/pytorch/pytorch/issues/26400
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27456
Differential Revision: D19300270
Pulled By: ezyang
fbshipit-source-id: ec87d225e23445020b377521e0daccceb4748215
