feat: add new callback hook on_checkpoint_write_end

[Iruos8805]

What does this PR do?

This PR introduces a new callback hook, on_checkpoint_write_end, which is triggered after a checkpoint file has been fully written to disk.

It allows users to run custom logic such as validation, integrity checks, or post save actions once checkpoint writing is complete.

Currently, on_save_checkpoint is triggered before the checkpoint file is written to disk.
However, there is no callback hook that runs after the checkpoint write operation is fully complete.

This limitation makes it difficult to safely perform actions that depend on the finalized checkpoint file, such as:

• Running integrity checks or file validations
• Launching asynchronous or distributed processes that use the newly written checkpoint
• Triggering external tools (uploading to remote storage, post-save logging)

The motivation behind this change is to enable developers to reliably run logic only after the checkpoint is guaranteed to exist on disk.

Fixes #15795

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📚 Documentation preview 📚: https://pytorch-lightning--21323.org.readthedocs.build/en/21323/

[bhimrazy]

Hi @Iruos8805,

I noticed there’s an after_save_checkpoint method defined in the logger:

pytorch-lightning/src/lightning/pytorch/loggers/logger.py

Lines 31 to 36 in 32442c2

	class Logger(FabricLogger, ABC):
	"""Base class for experiment loggers."""
	def after_save_checkpoint(self, checkpoint_callback: ModelCheckpoint) -> None:
	"""Called after model checkpoint callback saves a new checkpoint.

Would this method already address the issue being discussed here?