[fx] get rid of graph_module.root (#44092)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44092
instead submodules and weights are installed directly on the
graph_module by transferring the original modules. This makes it more
likely that scripting will succeed (since we no longer have submodules
that are not used in the trace). It also prevents layered transforms
from having to special case handling of the `root` module. GraphModules
can now be re-traced as part of the input to other transforms.
Test Plan: Imported from OSS
Reviewed By: jamesr66a
Differential Revision: D23504210
Pulled By: zdevito
fbshipit-source-id: f79e5c4cbfc52eb0ffb5d6ed89b37ce35a7dc467
