Failure as NNlib.within_grad is deleted

[mcabbott]

Flux's downstream tests see a failure with latest NNlib, because this line calls a function which no longer exists:

GeometricFlux.jl/src/operation.jl

Line 46 in c306989

dx = if NNlib.within_grad()

Removed in FluxML/NNlib.jl#434. One path would be just to upgrade here. Another would be to make an NNlib PR adding a deprecation for the removed function (which was assumed to be internal); this option would keep old versions of this package working with new NNlib.

[yuehhua]

@mcabbott Thank you so much for raising this issue!