Fix crash when old checkpoint does not exist anymore
jenkins This PR is stale because it has been open for 14 days with no activity. Remove stale label or comment or update or this will be closed in 7 days. This PR was closed because it has been inactive for 7 days since being marked as stale. Re-opening, can someone please help review this minor change? +1 on this issue Closing as this issue has been addressed in #8130
# What does this PR do ?

This prevents NeMo from crashing when trying to delete a checkpoint that has already been deleted.

This situation may happen for instance after manually fixing corrupted checkpoints.

Note that I believe PTL also has a similar safety guard: https://github.com/Lightning-AI/lightning/blob/119039b932c79f1e5daef67345628e675ab3c0bf/src/lightning/fabric/plugins/io/torch_io.py#L94

**Collection**: nlp

# Changelog 
- Fix crash when attempting to delete an old checkpoint that does not actually exist anymore

# Jenkins CI
To run Jenkins, a NeMo User with write access must comment `jenkins` on the PR.

# Before your PR is "Ready for review"
**Pre checks**:
- [X] Make sure you read and followed [Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md)
- [ ] Did you write any new necessary tests?
- [ ] Did you add or update any necessary documentation?
- [ ] Does the PR affect components that are optional to install? (Ex: Numba, Pynini, Apex etc)
  - [ ] Reviewer: Does the PR have correct import guards for all optional libraries?
  
**PR Type**:
- [ ] New Feature
- [X] Bugfix
- [ ] Documentation

## Who can review?

Anyone in the community is free to review the PR once the checks have passed. 
[Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md) contains specific people who can review PRs to various areas.