Implement Convolutions

[s-cavalier]

It doesn't appear that there is a standard Convolution library in C++ like there is for FFTs and matrix math. Thankfully, convolutions are just calls to FFTs.

Action items:

• Understand first how convolutions work with FFT. Some resources:
• 3b1b on Convolutions, check the last 5 min
• An implementation I made a couple years ago writing FFT and convolutions from scratch. It's two years old, so not super performant; try to focus on the high-level algorithm design for fftconvolve2d
• You can use some abstract or basic fft implementation as the subroutine. You can ChatGPT the FFT impl if you want, since it will later be replaced.
• Try to ensure everything is performant (avoid unnecessary allocs and retreading over memory). A PR should make an argument to the performance of the implementation.