Improve model checkpoint saving logic

[lucas-ventura]

Improve model checkpoint saving logic to always save model checkpoint when validation is not run

What does this PR do?

This PR enhances the model checkpoint saving logic to always save the model checkpoint when validation is not run. Previously, the model was only saved if validation was performed and the validation loss improved. With this change, we ensure that a checkpoint is saved after every epoch if validation is turned off (--run_validation False).

Please describe the tests that you ran to verify your changes and relevant result summary. Provide instructions so it can be reproduced.
Please also list any relevant details for your test configuration.

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