Include Keras tensor in the allowed types
Includes Keras tensor in the allowed types. This allows propagating symbolic `keras.Input` tensors through the models' `call` method. This way we can convert any HF (subclass) model into a functional model.

@Rocketknight1

@sergiovalmac looks like all the CI tests are passing except the code quality, let me run that one for you!<|||||>@sergiovalmac tests passing now, merge whenever you're happy with the PR!<|||||>Brilliant! Thank you very much! Please could you merge it? It says that only those with write access can merge PRs.<|||||>Ah, I'm sorry, I thought you should be able to once I approved it!<|||||>Thanks! :-)