def __init__(self, boxes, labels=None, fg_inds_wrt_gt=None):
        """
        Args:
            boxes: Nx4
            labels: N, each in [0, #class), the true label for each input box
            fg_inds_wrt_gt: #fg, each in [0, M)

        The last four arguments could be None when not training.
        """
        for k, v in locals().items():
            if k != 'self' and v is not None:
                setattr(self, k, v)

        Args:
            boxes: Nx4
            labels: N, each in [0, #class), the true label for each input box
            fg_inds_wrt_gt: #fg, each in [0, M)

        The last four arguments could be None when not training.
        
examples/FasterRCNN/modeling/model_frcnn.py