Benchmark against tensorflow's fft

[zaccharieramzi]

I am originally a user of keras with tensorflow backend. I am working with networks that need to use the fft (in particular FFT2D). It turns out that the fft in tensorflow is apparently inherently slow (cf: tensorflow/tensorflow#6541).

I wanted to know if someone had taken the time to benchmark them against each other. I think I will try to do it myself, but if someone has done it, I might as well use that.

[zaccharieramzi]

I guess since fft is in pytorch now I should benchmark this one.