[dynamo] Don't copy the graph during checkpointing (copy_graphstate) (#89232)
copy_graphstate is called a ton, this makes copy_graphstate a lot faster, helps with https://github.com/pytorch/torchdynamo/issues/1803
tag each graph node with a timestamp, when checkpointing store the timestamp, when restoring remove nodes older than the timestamp stored in the state. This essentially has the same behavior as the original impl, just doesn't copy the whole graph.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/89232
Approved by: https://github.com/jansel
