Support zero dimensions in softmax decompositions (#91322)
The eager implementation of softmax supports computation along zero dimensions, but many of the other implementations did not, including:
* decompositions & refs (this was causing dynamo failures)
* forward AD for logsumexp
* MPS log_softmax_backward
This PR handles the `input.numel() == 0` cases separately to avoid running `amax()`, which fails for zero dimensions, and updates opinfos.
example of "computation along zero dimensions":
```python
# example of where
import torch
t = torch.rand((4, 0, 0))
print("~")
print(torch.nn.functional.softmax(t, dim=-1)) # this passes
print("~")
torch._refs.softmax(t, dim=-1) # this fails
print("~")
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/91322
Approved by: https://github.com/lezcano
