<SFT> <Long Sequence Length> Remove Attention Mask in SFT Chat Dataset
# What does this PR do ?

This PR removes the attention mask generation to prevent CPU from running out of memory issue when using SFT chat dataset.

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**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.

