Mixed Precision in LuxLib rules

[willtebbutt]

@penelopeysm discovered in TuringLang/docs#559 (comment) that Mooncake's current rule for matrix-matrix multiplication in LuxLib doesn't successfully handle the case that the two input arrays contain numbers at different precisions.

It really shouldn't be too hard to handle this properly -- we would just need to re-write the current from_rrule implementations of the various variants of matrix-matrix multiplication found around here to have proper Mooncake rules which

1. define is_primitive for arrays whose elements are subtypes of IEEEFloat, and
2. define the rrule!s in such a way that ensures the correct element-type is adhered to.
3. add a method of rrule!! which is a catch-all for all other element types, which always errors with some kind of sensible error message that users can make use of to know how to modify their code.

Note: this is also a great opportunity to ensure excellent performance in Lux.jl -- the current implementations of the rules involve more allocations than are really needed, because we do not increment the gradients in-place.

[yebai]

Could this be a potential duplicate of #196? In that PR, I explicitly suggested supporting mixed precision, but I agree it is better to refrain from performing any promotion implicitly.

[willtebbutt]

Hmmm good point. I'll move this there.