Add optional index mapping dir in mmap text datasets
If datasets are stored on a read-only medium, index files cannot be created into adjacent files and an
alternative directory must be specified for index
mapping files.

# What does this PR do ?

This commit adds an optional flag to specify the directory to write index files to.

**Collection**: nlp

# Changelog 
- Add an optional `index_mapping_dir` to the constructors of `TextMemMapDataset`,  `CSVMemMapDataset` and `JSONLMemMapDataset`.
- Add unit tests.
- Propagate `index_mapping_dir` from `GPTSFTDataset`.
- 
# Usage

```bash
nvidia-docker run --shm-size=4gb  -v ~/llm/data:/llm/data:ro -it sft:latest  python megatron_gpt_sft.py ++model.data.train_ds.index_mapping_dir=/tmp  ++model.data.validation_ds.index_mapping_dir=/tmp
```

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- [x] Did you write any new necessary tests?
- [x] Did you add or update any necessary documentation?
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  - [ ] Reviewer: Does the PR have correct import guards for all optional libraries?
  
**PR Type**:
- [x] New Feature
- [ ] Bugfix
- [ ] Documentation

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# Additional Information
* Related to # (issue)
