Remove redundant sum() rules #803

Job	Run time
Flux.jl/All	26s
NNlib.jl/All	43m 3s
DynamicPPL.jl/All	25s
DistributionsAD.jl/Zygote	1m 36s
DiffEqFlux.jl/Layers	23s
DiffEqFlux.jl/BasicNeuralDE	22s
NeuralPDE.jl/NNPDE	25s
Molly.jl/Zygote	14m 59s
	1h 1m 39s

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