fix convert_llama_hf_to_nemo.py llama31 argument
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Your code was analyzed with PyLint. The following annotations have been identified:

```
************* Module convert_llama_hf_to_nemo
scripts/checkpoint_converters/convert_llama_hf_to_nemo.py:63:0: C0301: Line too long (218/119) (line-too-long)
scripts/checkpoint_converters/convert_llama_hf_to_nemo.py:46:0: C0116: Missing function or method docstring (missing-function-docstring)
scripts/checkpoint_converters/convert_llama_hf_to_nemo.py:75:0: C0116: Missing function or method docstring (missing-function-docstring)
scripts/checkpoint_converters/convert_llama_hf_to_nemo.py:129:0: C0116: Missing function or method docstring (missing-function-docstring)

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```

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# What does this PR do ?

Fix conversion script arguments. The original script using type=bool convert empty strings to False and non-empty strings to True. Might not be an ideal way of using it.

**Collection**: NLP

# Changelog 
- Fix convert_llama_hf_to_nemo script --llama31 arguments.

# Usage

```shell
$ scripts/checkpoint_converters/convert_llama_hf_to_nemo.py --llama31  # when it is llama31
$ scripts/checkpoint_converters/convert_llama_hf_to_nemo.py                  # when it is not llama31
```

