Fix mixtral_nemo_to_hf conversion
Is it fine & really required to introduce extra param `--input_tokenizer`?

If yes, should we update relevant docs? https://github.com/NVIDIA/NeMo/blob/main/docs/source/ckpt_converters/dev_guide.rst

CC @yaoyu-33 - please have a look, maybe you will have some comments > Is it fine & really required to introduce extra param --input_tokenizer?

I'm modeling this off #8000. In both cases, the problem is that the converter tries to use the default HuggingFace tokenizer, while the custom model that's being converted may well be using a different tokenizer.

Perhaps the correct solution in both cases is to unpack the tokenizer out of the .nemo file during the conversion script, instead of adding a `--input_tokenizer` argument. 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.
# What does this PR do ?

Fix convert_mixtral_nemo_to_hf.py

**Collection**: scripts/checkpoint_converters

# Changelog 
 * Fix two typos
 * Allow for custom tokenizer to be passed in

**PR Type**:
- [ ] New Feature
- [x] Bugfix
- [ ] Documentation

## Who can review?

@cuichenx 

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.
