Fix ShouldValidateCallback

[JohnGiorgi]

Addresses a bug in #5534.

Changes proposed in this pull request:

There were two subtle bugs in ShouldValidateCallback merged in #5534. One caused validation to always happen after the first epoch regardless of validation_start. The other caused validation to happen every validation_interval + 1 epochs instead of every validation_interval epochs. Fixed both with the following changes:

• Add on_start method to ShouldValidateCallback that correctly sets _should_validate_this_epoch when training begins.
• Because on_epoch is called at the end of every epoch, it needs to consider epoch + 1 (instead of epoch) when setting _should_validate_this_epoch.
• Update TrainerTest.test_should_validate_callback so that it fails without these fixes.

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[dirkgr]

Nice catch, and special thanks for adjusting the tests to catch this in the future!