How imagenet mean and std derived? To use pre-trained models it is a preferred practice to normalize the input images with imagenet standards.
mean=[0.485, 0.456, 0.406] and std=[0.229, 0.224, 0.225].
How are these parameters derived?
These are calculated based on millions of images of ImageNet.

Ref - SO
Ref - MachinelearningMastery