Faster AlphaBetaRule

[adrhill]

The AlphaBetaRule used to compute pullback functions on all four modified layers (layerᵅ⁺, layerᵅ⁻, layerᵝ⁻ and layerᵝ⁺) using Zygote.
However, since all layers are linear and layerᵅ⁺ & layerᵝ⁺ as well as layerᵅ⁻ & layerᵝ⁻ only differ in the bias terms, two pullbacks can be skipped as they are duplicates.

Local benchmarks

                                          ID                 time ratio memory ratio
  –––––––––––––––––––––––––––––––––––––––––– –––––––––––––––––––––––––– ––––––––––––
    ["apply rule", "AlphaBetaRule", "Dense"] 0.68 (5%) :whitecheckmark:    1.00 (1%)

[codecov-commenter]

Codecov Report

Merging #108 (781b21a) into master (9695738) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #108   +/-   ##
=======================================
  Coverage   94.70%   94.70%           
=======================================
  Files          16       16           
  Lines         529      529           
=======================================
  Hits          501      501           
  Misses         28       28           

Impacted Files	Coverage Δ
src/lrp/rules.jl	94.95% <100.00%> (ø)

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