Fix aot sequence_nr to reset bwd flag (#107210)
The way the aot autograd sequence_nr tracking works is that we run the aot export logic, the dynamo captured forward graph is run under an fx.Interpreter, which iterates through the nodes of the forward graph while setting the `current_metadata`.
Since during backward what is run doesn't correspond to any node during forward, we fallback to the global `current_metadata`. And since this global metadata is ends up being shared between runs, that leads to weirdness if we forget to reset things, e.g., depending whether this is the first test run, the printed results will be different.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/107210
Approved by: https://github.com/bdhirsh
