Find a way to get Zygote working with PEPS without overloading `Base.:+(::PEPS, ::PEPS)`

[mtfishman]

Currently, Zygote makes use of the definition Base.:+(::PEPS, ::PEPS) to add up sums of gradient terms that show up in the chain rule. For the sake of adding up gradient terms, it is an elementwise sum of the elements of the PEPS tensors. However, this definition is at odds with the usual definition we would think of for adding PEPS in terms of adding the quantum states.

It would be nice to find a way to overload Zygote behavior so we can define Base.:+(::PEPS, ::PEPS) as the addition of quantum states instead of elementwise addition.

Cross reference: https://github.com/ITensor/ITensorNetworkAD.jl/pull/10#discussion_r671600409