faketensor: prevent deepcopy from cloning FakeTensorMode (#104476)
fixes https://github.com/pytorch/pytorch/issues/104465
A more detailed repro is here, which uses `nn.TransformerLayer` (this breaks with AOTAutograd today, due to the presence of multiple FakeTensorMode objects lying around) https://github.com/pytorch/pytorch/issues/103505#issuecomment-1614817132
Pull Request resolved: https://github.com/pytorch/pytorch/pull/104476
Approved by: https://github.com/ezyang
