Add missing complex support for torch.norm and torch.linalg.norm (#48284)
Summary:
**BC-breaking note:**
Previously, when given a complex input, `torch.linalg.norm` and `torch.norm` would return a complex output. `torch.linalg.cond` would sometimes return a complex output and sometimes return a real output when given a complex input, depending on its `p` argument. This PR changes this behavior to match `numpy.linalg.norm` and `numpy.linalg.cond`, so that a complex input will result in the downgraded real number type, consistent with NumPy.
**PR Summary:**
The following cases were previously unsupported for complex inputs, and this commit adds support:
- Frobenius norm
- Norm order 2 (vector and matrix)
- CUDA vector norm
Part of https://github.com/pytorch/pytorch/issues/47833
Pull Request resolved: https://github.com/pytorch/pytorch/pull/48284
Reviewed By: H-Huang
Differential Revision: D25420880
Pulled By: mruberry
fbshipit-source-id: 11f6a2f3cad57d66476d30921c3f6ab8f3cd4017
