[Metrics] Add Image Gradient
🚀 Feature

Implement Image-Gradients for PT Lightning.
Motivation

Recently I was working on a vanilla PT implementation of the DenseDepth paper. They happen to use a DepthLoss as one of their loss functions. Incidentally, DepthLoss is based on calculating Image gradients between two images, native implementation of which doesn't exist in PT! Including Image Gradient as a metric would make life easier for devs working on similar papers/projects. Also often metrics like ssim and image gradients are used in conjunction (as in this paper)
Pitch

Make Image-gradients available as a metric out-of-box in PT Lightning
Alternatives

Alternative solutions often include custom implementation of these metrics, thus increasing the development overhead.