Fix last and best checkpoints issue

[HangJung97]

What does this PR do?

Fix last and best checkpoints issue:

• Lightning's save_last in ModelCheckpoint checkpoint does not actually save the last model but creates a symbolic link to the best weight. Hence, it is required to add another ModelCheckpoint callback that monitors epoch to save the latest checkpoint.
• Best checkpoint is now saved as best.ckpt and best_epoch_{number}.ckpt, while last checkpoint is saved as last.ckpt and latest_epoch_{number}.ckpt.

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