mul! implemented for 2d and 3d multiplication with NNlib

[ajozefiak]

The implementation could be made cleaner, but the general working idea is present in my branch.

[ChrisRackauckas]

Is there a test which hits this dispatch?

[ajozefiak]

@ChrisRackauckas I'll need to add a fewmore tests, but the tests I've created for the fallback in differentiation_dimension.jl hit the 3D case along with the fallback for higher dimensions. I will need to explicitly test the 2D case, but that is also being hit (for differentiation along rows) in a few places in derivative_operator_interface.jl