Fix typo in checkpoint init

[rybchuk]

There's a typo in the new checkpoint Callbacks, super().__init() instead of super().__init__(). When the Callbacks are run, they produce an error: AttributeError: 'super' object has no attribute '_ModelCheckpointCallback__init'. After this fix, I successfully ran a job and generated checkpoint files.

[JeanKossaifi]

Thanks @rybchuk! @dhpitt let's add some unit-tests for the callbacks to make sure they work as expected. We can use pytest to mock the save file.