Add DUCC FFT support

Damiano Franzòrequested to merge
damianofranzo/eigen:duccfft_backend into master

Reference issue

There isn't a specific reference issue for this feature.

What does this implement/fix?

This pull request adds support for using the DUCC FFT implementation within the Eigen unsupported module. This code is basically a very minor rework of a 2-year-old MR from @cantonios . In practice, I downloaded the .patch file from !1068 (closed) and worked out the details. The FFT tests passed successfully.

Please, let me know if this code requires further modification to be merged, or if you'd like to avoid this from being merged into Eigen at all.

Additional information

The main goal is to extend this support to Eigen tensors, and I'd volunteer to tackle it as a next step.

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