[NLP] Improve and unify loading state_dict for community models
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# What does this PR do ?

- Provide a common way for loading state_dict for HF models among different conversion scripts
- Deduplicate tokenizers in the Nemo file produced: the model now comes with a single copy instead of an extra redundant tokenizer file (which is an undesired side effect of [`load_model` function](https://github.com/NVIDIA/NeMo/blob/52d50e9e09a3e636d60535fd9882f3b3f32f92ad/scripts/nlp_language_modeling/convert_hf_llama_to_nemo.py#L58))

 
**Collection**: [NLP]

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- [x] Make sure you read and followed [Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md)
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**PR Type**:
- [x] Improvement

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# Additional Information
* Related to https://github.com/NVIDIA/NeMo/pull/7935
