Add precision arg in mixtral converter comment 
# What does this PR do ?

Update mixtral hf to nemo conveter comments via adding `--precision` arg


**Collection**: [NeMo/scripts/checkpoint_convters/convert_mixtral_hf_to_nemo.py]

# Changelog 

````
   Example to run this conversion script:
     python3 convert_mixtral_hf_to_nemo.py \
      --input_name_or_path <path_to_mixtral_checkpoints_folder> \
-     --output_path <path_to_output_nemo_file> 
+     --output_path <path_to_output_nemo_file> \
+    --precision=bf16
````

# Usage

```python
    python3 convert_mixtral_hf_to_nemo.py \
     --input_name_or_path <path_to_mixtral_checkpoints_folder> \
     --output_path <path_to_output_nemo_file> \
     --precision=bf16
```

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**PR Type**:
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- [ ] Bugfix
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@akoumpa @ericharper 

# Additional Information
* Related to # (issue)
