Add padding=same, support half-precision input

[papkov]

• Calculate padding size based on kernel for stride=1, dilation=1
• Convert half-precision signal and kernel to float, because rfftn does not support half at the moment
• Remove unnecessary signal copy in padding

[fkodom]

Thanks for this contribution! Just FYI -- I'll probably get to look over this in the next few days. Super busy lately haha 👍

[D1anskr]

If an RGB image is convoluted, is it 2D or 3D? Can there be a demonstration about fast convolution of pictures

[lukepolson]

Is this going to be merged?

[fkodom]

Hey folks -- really sorry about this. It somehow slipped through the cracks, and I forgot it was still open. 🙁

I agree this is a good contribution. All tests passing, and very clean implementation. 🎉