Replace GPT2TokenizerFast with tiktoken-based tokenizer (#44)

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This PR fixes issue #43 by replacing GPT2TokenizerFast with a tiktoken-based tokenizer. The new implementation supports both messages for chat completions models and simple strings for ordinary completions models.

List of commits:

1. Implement tiktoken-based tokenizer in autopr.utils.tokenizer
2. Update tokenizer usage in autopr/utils/repo.py
3. Update tokenizer usage in autopr/repos/completions_repo.py
4. Add tests for the new tiktoken-based tokenizer