Unify data sample converting function in MMDet and MMDet3D

[ZwwWayne]

In the base 3D detector, it uses convert_to_datasample shown as below https://github.com/open-mmlab/mmdetection3d/blob/f9e5e98f7b5625b147515224f7497b4b99a15aaa/mmdet3d/models/detectors/base.py#L92

However, in MMDet, it uses add_pred_to_datasample: https://github.com/open-mmlab/mmdetection/blob/c9b47820d637ab341ca3ca869116fc3d91030c74/mmdet/models/detectors/base.py#L130

We should unify the name among them.