[Agent] Added screenshot_dir and lazy loading of MLX

[ddupont808]

This PR reduces the output of saving the trajectories from full OSWorld evals from >100gb to <2gb by adding a screenshot_dir parameter

Usage

ComputerAgent(
        model="..."
        trajectory_dir={
            "trajectory_dir": f"trajectories/osworld_{job_uuid}",
            "screenshot_dir": "trajectories/call_screenshots"
        },
)