vLLM Export Improvements
# What does this PR do ?

Adds support for LLAMA3 and fixes a few other issues with vLLM.

**Collection**: NLP

# Changelog 
- Separated the vLLM export functionality from the common deployment script into deploy_vllm_triton.py.
- Fixed LLAMA3 exprot (wrong vocab_size)
- Improved the export test.

**PR Type**:
- [X] Bugfix
