(Anisotropic) Fourier Feature Positional Encodings

[NabJa]

I would like to contribute specialized positional encodings. This issue is meant to track the request and gather feedback from the community and maintainers before I open a pull request with the implementation.

Is your feature request related to a problem? Please describe.
Sinusoidal and Learned positional encodings (PEs) are often suboptimal for medical images. This is especially the case for anistropic images and for videos (see pre-print accepted in ShapeMI@MICCAI here).

Describe the solution you'd like
I would like to add the positional encodings, described in the pre-print:

1. Isotropic Fourier Feature Positional Encodings (IFPE)
2. Anistropic Fourier Feature Positional Encodings (AFPE)

They could be implemented in a single function and used in the PatchEmbeddingBlock.

Describe alternatives you've considered
Alternatively the PEs would have to be imported from another library.