[functorch] Don't unnecessarily wrap the elem in PythonTensor (pytorch/functorch#554)
* Don't unnecessarily wrap the elem in PythonTensor
Instead of saying that a PythonTensor has a regular (e.g., CPU) tensor
and an FX proxy, a PythonTensor *is a* regular CPU tensor, that also
carries an FX proxy (that updates as we go along).
This should fix https://github.com/pytorch/functorch/issues/465 and
it also fixed some expected failures in the test suite.
This kills the meta variant logic entirely; maybe some other time we'll
try to bring it back.
Signed-off-by: Edward Z. Yang <ezyang@fb.com>
