keras/src/layers/merging/subtract.py/Subtract/_merge_function
class Subtract:    def _merge_function(self, inputs):
        if len(inputs) != 2:
            raise ValueError(
                "A `Subtract` layer should be called on exactly 2 inputs. "
                f"Received: inputs={inputs}"
            )
        return ops.subtract(inputs[0], inputs[1])