foreground RoIs selection

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Select foreground RoIs as those with >= FG_THRESH overlap

fg_inds_first = np.where(max_overlaps >= cfg.TRAIN.FG_THRESH)[0]
fg_inds = []
for n in fg_inds_first:
    if indices[n] in gt_max or n in gt_indices[:, 0]:
        fg_inds.append(n)
    else:
        labels[n] = 0.  #why labels[n] is set be 0 here, is there means idx==n selected as a background one?