Fix should validate callback train end

[JohnGiorgi]

Addresses another bug in #5534. Sorry my original PR had so many bugs in it!

Changes proposed in this pull request:

The ShouldValidateCallback merged in #5534 was missing the method on_end. This caused validation to never occur at the end of training, even if the final epoch satisfied validation_start and validation_interval. I added an additional check to the test for this scenario.

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