[functorch] Apply functorch-side fix for PythonTLSSnapshot change (pytorch/functorch#633)
We add PythonTLSSnapshot to the local exclude while functorch is active
and remove it when we leave functorch. This is a quick fix; there's
something nicer we can do (which is refactor DynamicLayer to not have so
many RAII guards that modify the thread-local dispatcher state)
