[functorch] Added has_aux argument to jacfwd and jvp (pytorch/functorch#419)
* Enabled non-tensor output for jvp, jacfwd
* Added jacfwd tests
* Do not accept functions returning non-tensor output
* WIP
* Updated tests and vjp output checking
* Added has_aux to jacfwd and jvp
* Enabled non-tensor output for jvp, jacfwd
* Added jacfwd tests
* Do not accept functions returning non-tensor output
* Updated tests and vjp output checking
* Updates according to the review and
assert_non_empty_output -> assert_non_empty_tensor_output
* Added regexps
* More updates
* Added tests for unacceptable aux type and tests for grad has_aux=True
* Fixed issue with test_grad_aux_pytree
* Put _undo_create_differentiable and return into try/except clause
