add path planning task

[jyizheng]

This pull request introduces a new "Path Planning Task" for visual reasoning, where the goal is to find a path from a start to a goal position while avoiding obstacles. The changes include documentation updates, a new script for dataset generation, and the integration of new task-related classes into the package.

Path Planning Task Integration

• Added a detailed description and usage instructions for the new path planning task to the README.md, including configuration options and output structure.
• Introduced a new script, examples/generate_path_planning.py, which enables users to generate datasets for the path planning task with customizable parameters such as grid size, obstacle density, and output options.

Codebase Updates for Task Support

• Updated src/__init__.py to import and expose PathPlanningConfig, PathPlanningTask, PathPlanningGenerator, and get_path_planning_prompt, integrating the new task into the package's public API.