[functorch] Make functorch codegen more robust (pytorch/functorch#693)
Previously the functorch codegen would only work if you didn't have a
PyTorch develop install in your environment. This PR changes it so that
the functorch codegen works when you have a PyTorch develop install in
the environment.
The reason for this change is that the PyTorch develop install adds a
`tools` module into the environment. It turns out we can just rely on
the tools module and this makes our codegen more robust to changes to
pytorch/pytorch codegen (when compared to what we were doing before,
which was keeping a copy of the PyTorch codegen inside of the functorch
repo).
Test Plan:
- wait for tests
