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

[pranotosh2]

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