Fix guard for SUPPORTED_NODES (#120798)
The special-case code for handling SUPPORTED_NODES was producing a guard that looked like:
```
"G['torch'].utils._pytree.SUPPORTED_NODES[<class '__main__.CausalLMOutputWithPast'>].type"
```
resulting in a eval error trying to evaluate the guard.
This change adds a new source type (`ClassSource`) which is given a class type (in this case `CausalLMOutputWithPast`) and attempts to fetch it from its defining module. It then uses that to build the `SUPPORTED_NODES` guards instead of referring to the type directly.
Also added a unit test which fails before this change and passes after.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/120798
Approved by: https://github.com/anijain2305
