Add tracing context, Integrate dynamo guards into torch._guards (#90647)
As defined here: https://docs.google.com/document/d/1oniZEgAaHE1IMByPRWRKbUHeaW06E2HMfCTCQyMRLek/edit#
This PR creates a new structure, a TracingContext, whose lifecycle matches that of the traced frame. It carries on it a GuardsContext, and eventually, a FakeTensorMode. It is the source of truth of all accumulated guards.
In this PR, we create the structure, and integrate it into dynamo. We do so by mapping OutputGraph's guards structure to its guard structure.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/90647
Approved by: https://github.com/ezyang
