Adding support for Neg

[dstarkenburg]

Adding support for the Neg layer of ONNX.

PR Checklist

• Tests are added
• Documentation, if applicable

[dfdx]

Why not just -x (or .-x to be consistent with the code above)?

If you are worried about intersection with sub(xs...), me can modify all 3 functions above like this:

add(x, xs...) = .+(x, xs...)
sub(x, xs...) = .-(x, xs...)
neg(x) = -x

[dstarkenburg]

Would the way we save the function on the tape be ok with both sub and neg having the same operator, i.e .-(x) .-(xs...)

[dfdx]

In ONNX, we have Sub(A, B) and Neg(A) - two functions with different names and different number of arguments.

In Julia, we have A - B and -A - two different methods with the same name, but still easily distinguishable by the number of arguments.

There are a few ambiguous corner cases, but in normal flow - when you use - in Julia and Subs/Neg in ONNX - these operations have perfect 1-to-1 mapping. So I don't see a reason to worry.