langchain/memory/combined.py/CombinedMemory/load_memory_variables
class CombinedMemory:    def load_memory_variables(self, inputs: Dict[str, Any]) -> Dict[str, str]:
        """Load all vars from sub-memories."""
        memory_data: Dict[str, Any] = {}

        # Collect vars from all sub-memories
        for memory in self.memories:
            data = memory.load_memory_variables(inputs)
            memory_data = {
                **memory_data,
                **data,
            }

        return memory_data