feat: Asynchronous episode saving for faster recording

[0xNOY]

What this does

This PR implements asynchronous episode saving to reduce the delay (e.g., videos encoding, stats caluculating) between episodes during recording. It adds a new AsyncEpisodeSaver class and an async_saving flag to RecordConfig. When enabled, the episode saving process happens in a separate thread, allowing the robot/teleoperator to be reset or ready for the next episode immediately.

How it was tested

I tested this on a real SO-101 using lerobot-record with --async-saving=true. I confirmed that the data was collected and saved correctly.

[Copilot]

Pull request overview

This PR implements asynchronous episode saving to reduce delays between episodes during robot recording. The key change is adding a AsyncEpisodeSaver class that saves episodes in a background thread, allowing the robot/teleoperator to be reset immediately after data collection without waiting for video encoding and stats computation.

Key Changes

• Added AsyncEpisodeSaver class that uses a Queue and background Thread to save episodes asynchronously
• Added async_saving boolean flag to RecordConfig to enable/disable the feature
• Modified the record() function to conditionally use async saving based on the configuration

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[Copilot]

The new AsyncEpisodeSaver class and async_saving feature lack test coverage. The existing test_record_and_resume in tests/test_control_robot.py should be extended to test async_saving=True to ensure the feature works correctly and doesn't break existing functionality. Consider adding tests that verify: 1) episodes are saved correctly with async_saving enabled, 2) wait_for_stop() completes all pending saves, and 3) error handling works properly.