Support fx_pytree in dynamo (#105574)
This PR does two things:
1. Make dynamo trace through fx_pytree (on top of torch.utils._pytree) so that generated graph modules can be retraced.
2. Fix bug where unflatten not returning dynamo VariableTracker.
Differential Revision: [D47734623](https://our.internmc.facebook.com/intern/diff/D47734623)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/105574
Approved by: https://github.com/yanboliang, https://github.com/ydwu4
