📝 Add typing to some callback classes

[gabrieldemarmiesse]

Provides an exemple to keras-team/tf-keras#555

[fchollet]

Thanks for the PR! First, we'll discuss with the team to determine whether we need/want explicit types throughout the codebase.

[fchollet]

In general we should avoid explicit types when the expect type is a custom class, like in the above case. It creates significant tech debt and maintenance burden going forward.

[fchollet]

After extensive discussions, this is what we will do going forward:

We accept type annotations for simple Python types (not unions, and not custom types), but they're optional when adding new code. We will merge contributor PRs that add such annotations going forward. We will not maintain custom types.

Please update the PR accordingly and format the code using black. Thanks!

[gabrieldemarmiesse]

Could we make an exception for the public api? I get that it's a lot of work to use custom types everywhere in the codebase and to maintain them, but for the public API, it has a direct benefit to the user through Mypy and IDEs, so it might outweight the cost of maintenance

Furthermore, those public API type hints could be in theory be read and displayed in the documentation, giving us even more incentive to do it

This move would be in line with the first of the keras-team values: We have empathy for our users

[fchollet]

This is our policy at this time. We may revise it in the future.

[fchollet]

Thanks for the updates!