Fix import (#861)

tools/analysis_tools/benchmark.py

@@ -3,11 +3,10 @@ import time
 import torch
 from mmcv import Config
 from mmcv.parallel import MMDataParallel
-from mmcv.runner import load_checkpoint
+from mmcv.runner import load_checkpoint, wrap_fp16_model
 
 from mmdet3d.datasets import build_dataloader, build_dataset
 from mmdet3d.models import build_detector
-from mmdet.core import wrap_fp16_model
 from tools.misc.fuse_conv_bn import fuse_module