Improve `torch.norm` functionality, errors, and tests (#41956)
Summary:
**BC-Breaking Note:**
BC breaking changes in the case where keepdim=True. Before this change, when calling `torch.norm` with keepdim=True and p='fro' or p=number, leaving all other optional arguments as their default values, the keepdim argument would be ignored. Also, any time `torch.norm` was called with p='nuc', the result would have one fewer dimension than the input, and the dimensions could be out of order depending on which dimensions were being reduced. After the change, for each of these cases, the result has the same number and order of dimensions as the input.
**PR Summary:**
* Fix keepdim behavior
* Throw descriptive errors for unsupported sparse norm args
* Increase unit test coverage for these cases and for complex inputs
These changes were taken from part of PR https://github.com/pytorch/pytorch/issues/40924. That PR is not going to be merged because it overrides `torch.norm`'s interface, which we want to avoid. But these improvements are still useful.
Issue https://github.com/pytorch/pytorch/issues/24802
Pull Request resolved: https://github.com/pytorch/pytorch/pull/41956
Reviewed By: albanD
Differential Revision: D22837455
Pulled By: mruberry
fbshipit-source-id: 509ecabfa63b93737996f48a58c7188b005b7217
