Buckets together trace frames that have the same caller:caller_port.

def _group_trace_frames(
        self, trace_frames: Iterable[TraceFrameQueryResult], limit: int
    ) -> Dict[Tuple[str, str], List[TraceFrameQueryResult]]:
        """Buckets together trace frames that have the same caller:caller_port.
        """
        # pyre-fixme[9]: caller_buckets has type `DefaultDict[Tuple[str, str], List[T...
        caller_buckets: DefaultDict[
            Tuple[str, str], List[TraceFrameQueryResult]
        ] = defaultdict(list)
        for trace_frame in itertools.islice(trace_frames, limit):
            caller_buckets[(trace_frame.caller, trace_frame.caller_port)].append(
                trace_frame
            )
        return caller_buckets