Fix for derivative of sinc(x) when x is positive but very very small (#56986)
Summary:
Problem arises for sinc'(x) where x != 0, but x ** 2 == 0, which happens for some very small floats.
I realized that my solution from https://github.com/pytorch/pytorch/issues/56763 was incomplete when I did a quick implementation using `torch.autograd.Function` and still got a `NaN` from my derivative.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/56986
Reviewed By: gchanan
Differential Revision: D28093507
Pulled By: albanD
fbshipit-source-id: 2a30e1065b08c5c60de843a0778dedeb0fb295f4
