[functorch] Update tests and examples to use make_functional*_v2
make_functional*_v2 is superior to the older make_functional. This PR
has all of our examples use it.
This PR also adds a "combine_state_for_ensemble(models)" API.
Coming soon: We're probably going to break BC on make_functional and
replace it with make_functional_v2. That's the nice thing about being a
prototype, we don't have to worry about BC too much.
