FixTextMemMapDataset index file creation in multi-node setup
# What does this PR do ?

Fixes issue #6715 by creating index files on all processes with local_rank 0.

**Collection**: NLP

# Changelog 
- Update rank condition to create index files when local_rank is 0 (formerly when global_rank was 0).

# Usage
* No change in used.

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**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. 
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# Additional Information
* Related to #6715 
