Add signatures to the return value of tf.saved_model.load()

Only works with models exported from tf.saved_model.save(). We still need a different path for restoring other SavedModels, but hopefully we can make the signature representations identical (keywords in, dict out).

In order to make this work I had to wrap exported signatures in another layer of function. I should either clean that up, or more likely we should come up with an easy API for inspecting the graphs of nested function calls.

PiperOrigin-RevId: 232742842