fix PSNR error for averaging all batches

[Gancheekim]

Hi, thanks for the great work here! However, i have noticed some thing weird when using PSNR() class.

the original implementation computes MSE over all batches of image, and calculate the PSNR over it: 10log_10(255/MSE_avg), where MSE_avg = mean(MSE) over [B, C, H, W]

to calculate the mean PSNR, it should be: mean(10log_10(255/MSE_i)) over each batch, where MSE_i is MSE over each [C,H,W]

notice that the 2 approaches are: compute mean inside and outside log function, which is not equivalent in terms of math.

[Gancheekim]

@styler00dollar

[styler00dollar]

I didn't see this pr until I randomly checked this repo, thanks.