Refactor RequestGenerator to use threading and update test suite

[markurtz]

Refactored RequestGenerator class:

• Replaced asyncio.Queue with Queue from the queue module for thread safety.
• Utilized threading for background queue population to ensure non-blocking request generation.
• Removed the start method as threading automatically starts the background task in async mode.
• Ensured that the _populate_queue method runs in a background thread to keep the queue populated.
• Implemented clean shutdown with the stop method joining the background thread. Updated unit tests:
• Added test_request_generator_sync_constructor and test_request_generator_async_constructor to verify constructor behavior.
• Added tests for repr and iter methods.
• Added tests to ensure create_item raises NotImplementedError if not overridden.
• Added tests to verify iter calls create_item the expected number of times.

Separated test files:

• Created tests/unit/request/test_base.py for unit tests.
• Created tests/integration/request/test_base.py for integration tests.

Unit tests:

• Verified the construction of the class with different input parameters.
• Mocked AutoTokenizer for testing tokenizer initialization with both a class implementation and a string alias.
• Ensured that the iter method works correctly in both sync and async modes.
• Verified that create_item is called the expected number of times.

Integration tests:

• Tested tokenizer construction with both a Hugging Face tokenizer and a string alias, ensuring the correct tokenizer is created.

[parfeniuklits]

Great job!

[parfeniukink]

@markurtz Might be not related to this PR, but actually the schedule package also contains dependencies on asyncio in its Task