Some basic svd forward rules and tests

[kshyatt]

Definitely not optimized...

[kshyatt]

Took another look at this. The Enzyme tests seem to be failing because of finite differences, for svd_full for example when size(A) == (17, 19), the finite differences result for dU is all over the place compared to that from the rule for the section of U which is only present in the full case. I'll try to think of a nice way to handle this, I think it is not occurring for Mooncake because Mooncake uses a different technique for checking against FD.

[lkdvos]

I think in general we only need the diagonal, so it might make sense to go the other way around: view(diagview(ΔS), 1:r)

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