add: image quality metrics

[yjy415]

Including:

• FID
• CLIP Score
• PickScore
• ImageReward
• HPSv2
• HPSv3
• Aesthetic

[gemini-code-assist]

Code Review

This pull request introduces a comprehensive suite of image quality evaluation metrics and reward models within the diffsynth.metrics module, including PickScore, ImageReward, HPSv2, HPSv3, CLIP, Aesthetic, and FID. These additions include the necessary model architectures, pre-trained weight loading logic, and utility functions for dataset handling, specifically for FID reference data. Documentation and example scripts were also provided for each metric. Review feedback correctly identified that the metrics overview table in the English documentation contained incorrect external links instead of relative paths to the example code, which should be corrected.