Implement gradient for QR decomposition

[jessegrabowski]

Description

QR is one of the few remaining linalg ops that is missing a gradient. JAX code for the jvp is here, whic also includes this derivation. This paper also claims to derive the gradients for QR, but I find it unreadable.

Relatedly but perhaps worthy of a separate issue, this paper derives gradients for the LQ decomposition, $$A = LQ$$, where $L$ is lower triangular and $Q$ is orthonormal ($$Q^TQ=I$$.) Compare this to QR, which gives you $$A = QR$$, where $$Q$$ is again orthonormal, but $$R$$ is upper triangular, and you see why I mention it in this issue. It wouldn't be hard to offer LQ as well.