Restore checkpoint save feature for 25.07 Nemo

[bdubauski]

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What does this PR do ?

It is no longer possible to use direct trainer.enable_checkpointing flag to enable checkpoints. Instead the ModelCheckpoint callback must be added to the recipe.

Collection: [Note which collection this PR will affect]

Changelog

Add ModelCheckpoint callback when save_checkpoint is true

Usage

• You can potentially add a usage example below

# Add a code snippet demonstrating how to use this 

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Additional Information

• Related to # (issue)