fix: validate empty generation batches

[danyalahmed1995]

Summary

Adds explicit handling for empty completion batches in the generation APIs.

text_completion([]) and chat_completion([]) previously reached lower-level generation logic and raised ValueError: min() arg is an empty sequence. chat_completion([[]]) could also fail with an IndexError when accessing the first dialog message.

This makes empty request batches return an empty result and raises a clearer ValueError for empty dialogs inside a chat batch.

Test Plan

• Ran python -m py_compile example_chat_completion.py example_text_completion.py
• Ran explicit py_compile validation for files under llama/
• Ran a no-weights validation snippet with a fake model/tokenizer for empty batch inputs

[jdasilvareis123-lab]

José da Silva Reis