breaking: make instance_type optional for Airflow model configs

[laurenyu]

Issue #, if available:

Description of changes:
This follows the work from #1567. It doesn't make sense to require instance_type in these two Airflow methods when it's not always used.

For the migration tool, I thought about not including model_config() on its own since there's a Keras module named model_config, but I couldn't find any function names that might collide. (Also, I tried searching GitHub to see what might be common ways of importing the function, but mostly just found people's forks/installations of our code.)

Testing done:
unit tests, linters

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[metrizable]

looks good to me

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