Merge branch 'add-model-docstring' into 'master'

Reformat docstrings in code

See merge request open-mmlab/mmdet.3d!115

tools/train.py

 from __future__ import division
+
 import argparse
 import copy
 import logging
+import mmcv
 import os
-import os.path as osp
 import time
-
-import mmcv
 import torch
 from mmcv import Config, DictAction
 from mmcv.runner import init_dist
+from os import path as osp
 
 from mmdet3d import __version__
 from mmdet3d.datasets import build_dataset