Fix error in examples of `torch.linalg.lu_factor` (#120484)
Found an error in the doc of `torch.linalg.lu_factor` related to `torch.linalg.lu_solve`. Also fix a sphinx issue by the way.
```Python traceback
TypeError: linalg_lu_solve(): argument 'LU' (position 1) must be Tensor, not torch.return_types.linalg_lu_factor
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/120484
Approved by: https://github.com/lezcano
