Refactor OutputGraph (#99987)
This PR splits OutputGraph into two classes:
- SubgraphTracer (handles FX-tracing)
- OutputGraph (handles Dynamo-specific output graph logic, like
tracking graph inputs, compiling the graph, and executing it).
The motivation behind this is in the next PR up in the stack.
TL;DR is: in order to do higher-order operators, we need nested
SubgraphTracer, one for each level of nesting of the higher-order
operators.
I'm happy to flatten the stack into a single PR, but this separate made
it easier for me to test. Lmk if you want the stack flattened.
Test Plan:
- existing tests
Pull Request resolved: https://github.com/pytorch/pytorch/pull/99987
Approved by: https://github.com/anijain2305, https://github.com/voznesenskym
