keras/src/ops/operation.py/Operation/input
class Operation:    def input(self):
        """Retrieves the input tensor(s) of a symbolic operation.

        Only returns the tensor(s) corresponding to the *first time*
        the operation was called.

        Returns:
            Input tensor or list of input tensors.
        """
        return self._get_node_attribute_at_index(0, "input_tensors", "input")