CUDAGraphs in Thunder

Nobody has claimed this yet.

PR #977 proposes to make CUDAGraphs a Transform with the cudagraphs being created in the `post_optimization` phase of creating the executable. This in particular enables people to subclass or copy the CUDAGraphs transform to have their own, bespoke thing.

We do have some ideas (and this is just a list from the top of my head to get things started), but this issue is to collect discussion and ideas about how you want CUDAGraphs to look like.

## static input detection

Currently we auto-mark parameters, but might be able to do more, e.g. buffers.
If we knew things about the things between fusions, we might know if we can re-use input buffers.

## caching

In particular we would not want to use a global cache, but a per-transform one, to let things go out of scope.
Also we want to enable users to potentially override the caching (we found that it can be interesting to re-use cudagraph fusions in case we have some non-grapheable bit in a repeated block of a model).
A step here is PR #1001

## operator selection for what is cuda-graphable

I think it would be cool if symbols in general had a tag whether they are suitable for CUDAGraph inclusion.
After #977 we have the advantage that if people have particular ideas about this, they can just subclass and override can_fuse.

## check composability

Test eg. fsdp + cudagraph
I'm guessing the communication prims don't mix well with cudagraphs? but that might be with operator tagging...

## (not immediate) future: memory information about ops

CUDAGraphs could enormously benefit by more information about the memory effects of operators (so similar to meta info but about what allocations a function makes, where the output is allocated etc.), but that is in the future.

Obiviously other people @nikitaved @IvanYashchuk @tfogal and more will be much more knowledgeable, but so here is a start.

Start by reading PR #977, then review the caching direction in PR #1001 and the issue's topics on static input detection, operator selection, and composability. The issue does not name files or tests, and completion is not defined; progress would require an agreed design and a concrete follow-up scope.

Written by the indexing model from the issue text.