Add standard evaluation metrics (MAE, MSE, PSNR, SSIM, FID, etc.)

Dear Authors,

I am currently working with this repository and facing difficulties while calculating evaluation metrics such as MAE, PSNR, SSIM, and others.

Request: It would be very helpful if you could include a standard utility script or module for metric calculation, covering commonly used metrics like:

MAE (Mean Absolute Error) MSE (Mean Squared Error) PSNR (Peak Signal-to-Noise Ratio) SSIM / MS-SSIM Any other relevant metrics

This addition would greatly improve usability and help ensure consistency across different implementations.

Thanking You Best regards