Add 3D SSIM

[lrlunin]

🚀 Feature

I do experiemnts with the 3D MRI scans and would like to add the SSIM metric for the 3D structural data.

The skimage.metrics.structural_similarity works for N-dimensional data and still can be use as a reference. I believe that the implementation for the N-dimensional case for pytorch would not pay off and the 2D and 3D cases are enough.

P.S. I already prepared a PR for this issue.