inline bias_act! fallback method

[ancapdev]

This may be a bit niche, but I build very small allocation free models with Flux and StaticArrays. bias_act!() is called from various Flux layers and appears sensitive to inlining heuristics. Beyond some level of complexity (size of inputs and type of activation) it allocates.

Example:

N = 32
X = rand(SVector{N, Float32})
b = rand(SVector{N, Float32})
@btime bias_act!(leakyrelu, $(Ref(X))[], $(Ref(b))[]);

Before:

 386.302 ns (5 allocations: 800 bytes)

After:

  2.988 ns (0 allocations: 0 bytes)