LRs updates are called at the end of a skipped epoch (#21307)

* fix: Update lr if train_batch_start returns -1

* fix: Rename variable: should_skip_rest_of_epoch

* chore: Update changelog

* fix: Batch increment

* Apply suggestion from @SkafteNicki

* test: Check lr is updated at the end of epoch

When `on_train_batch_start` returns -1, the rest of the epoch is skipped.
The lr update should still happen at the end of the epoch.
- Test is_last_batch has been set correctly
- Test lr has been updated at the end of each epoch

* doc: Add documentation for lr update

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Co-authored-by: Nicki Skafte Detlefsen <[email protected]>