Replace unsqueeze transform with stack (#101766)
As part of split-cat transforms, we needed to unsqueeze additional inputs (not coming from split) but going to the cat/stack nodes.
However, this leads to patterns like:
```
split -> unsqueeze -> cat
```
when there are multiple splits going into cat.
An alternative is to use stack rather than unsqueeze, leading to patterns like:
```
split -> stack -> cat
```
This is much better, as repeated applications of the same pattern will further simplify "split->stack", which is not trivial in case of "split->unsqueeze->cat".
Another nice side-effect is lesser number of nodes in the graph overall.
Differential Revision: [D45952452](https://our.internmc.facebook.com/intern/diff/D45952452/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/101766
Approved by: https://github.com/jansel
