[jax2tf] Added a test for SavedModel with pytrees
[jax2tf] Added a test for SavedModel with pytrees

This was in response to an external user running into problems
with converting and saving a SavedModel for a function where
one of the arguments is a dict. This test demonstrates that
this works, both for the conversion, and for the saving/restoring.

google/jax,google/jax,google/jax,google/jax