feat: support multiple completions

[BaiMoHan]

This PR adds support for multiple completions (n > 1) and improves the QPS handling mechanism. The changes include:

1. Support for multiple completions:

   • Remove assertion that limited choices to single completion
   • Add support for handling multiple completions when n > 1
   • Concatenate multiple completion texts with newlines
   • Use logprobs from first completion for metrics

2. Fix token count validation:

   • Fix AttributeError by using self.environment.parsed_options
   • Update token count validation to properly handle n > 1 cases
   • Improve warning message to show max_tokens and n values

Testing:

• Tested with n=4 completions
• Verified token count validation