Fix fusion replacement ordering#2150
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jessegrabowski
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Is there anything in here that should be generalized to toposort in general, not just the elemwise fusion logic?
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Apparently sequential toposort is not as trivial as I hoped for (even knowing there is some specialized literature out there on it). Should have LEAN-proofed my algorithm hehe
On the positive side I learned that to correctly replace in topological order in PyTensor it's enough to sort directly connected nodes, no need to perform an absolute toposort of all replaced nodes, because indirect nodes will hold the right replacements due to how inplace replacement works.
Closes #2145