For transformer signatures with a single input, add ctor for single KeyValue
This avoids having to put a { } around the KeyValue to construct an std::array<KeyValue,1> in-situ. This is now done by the new ctor which delegates to the old one.
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This avoids having to put a { } around the KeyValue to construct an std::array<KeyValue,1> in-situ. This is now done by the new ctor which delegates to the old one.