feat: add val_split to fit interface

[guenthermi]

Add val_split to fit interface

This PR adds the val_split parameter introduced by https://github.com/jina-ai/finetuner-core/pull/401 to the fit function

Example usage:

def fit(
    model,
    train_data,,
    validation_split = 0.2
)

• This PR references an open issue https://github.com/jina-ai/finetuner-core/issues/404
• I have added a line about this change to CHANGELOG

[guenthermi]

I am not sure when to merge this. At the moment it will not work, since it requires a release of a new stubs version