Train a real smaller teacher to be used in CTranslate2

[gregtatum]

In #922 I'm introducing a CTranslate2 test that runs a real teacher. The teacher is ~700MB, and takes 40 seconds to run. It would be nice to have a smaller teacher that can be used for testing, but that will produce real results so that we have confidence the setup is working.

The work here would be to define a smaller teacher model, and train it on a real run. This could be done as a small run while doing a bigger training.