Fix mistakes and increase clarity of norm documentation (#42696)
Summary:
* Removes incorrect statement that "the vector norm will be applied to the last dimension".
* More clearly describe each different combination of `p`, `ord`, and input size.
* Moves norm tests from `test/test_torch.py` to `test/test_linalg.py`
* Adds test ensuring that `p='fro'` and `p=2` give same results for mutually valid inputs
Fixes https://github.com/pytorch/pytorch/issues/41388
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42696
Reviewed By: bwasti
Differential Revision: D23876862
Pulled By: mruberry
fbshipit-source-id: 36f33ccb6706d5fe13f6acf3de8ae14d7fbdff85
