More granular callbacks

[awaelchli]

🚀 Make callback system more granular

Motivation

I am currently implementing #765 (make progress bar into a callback) and I need additional callback methods to do this.

Pitch

introduce these new callback methods:

• on_train_batch_start (currently named on_batch_start)
• on_train_batch_end (currently named on_batch_end)
• on_val_batch_start
• on_val_batch_end
• on_test_batch_start
• on_test_batch_end

and make on_batch_start run on any of the above *_start (same for on_batch_end)

Further suggestions:

• introduce on_train_epoch_start, on_val_epoch_start, on_test_epoch_start and corresponding *_end methods.

Alternatives

Keep as is, but I don't know how to implement the progress bar callback otherwise for validation/test updates.

[awaelchli]

For the record, #1450 introduced callbacks for validation_start/end, test_start/end, and sanity_check_start/end.

The question remains whether we should rename on_batch_* to on_training_batch_* for consistency with the naming of the others.

[Borda]

The question remains whether we should rename on_batch_* to on_training_batch_* for consistency with the naming of the others.

that it shall be for all batch train/valid...

[stale]

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