Add an initial integration test using model.fit

[mattdangerw]

We should add an integration test, running some actual limited training job for TransformerEncoder/TransformerDecoder, and possibly also using a tokenizer and position embedding.

[mattdangerw]

There's a lot more we could do here (e.g. tf.distribute testing), but a simple CPU integration test is probably a good start.

@chenmoneygithub assigning you as I think you were already scoping this out?