FFT parallelization threshold

[Pratyush]

Description

Adds a threshold when parallelizing FFTs. Currently the threshold is derived empirically; it gives speedups of between 15-30% compared to the non-threshold version.

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[weikengchen]

I think it looks good. But, a comment noting that this number is chosen empirically in the code would be helpful I think.

[Pratyush]

Ok, done!

[ValarDragon]

Thanks for integrating this! We should still benchmark this against hardcoding the base case tho