[Doc] SUNRGBD dataset doc update (#391)

* sunrgbd doc update and change in image path

* rephrase

data/sunrgbd/README.md

@@ -22,6 +22,11 @@ python tools/create_data.py sunrgbd --root-path ./data/sunrgbd  --out-dir ./data
 
 NOTE: SUNRGBDtoolbox.zip should have MD5 hash `18d22e1761d36352f37232cba102f91f` (you can check the hash with `md5 SUNRGBDtoolbox.zip` on Mac OS or `md5sum SUNRGBDtoolbox.zip` on Linux)
 
+NOTE: If you would like to play around with [ImVoteNet](../../configs/imvotenet/README.md), the image data (`./data/sunrgbd/sunrgbd_trainval/image`) are required. If you pre-processed the data before mmdet3d version 0.12.0, please pre-process the data again due to some updates in data pre-processing
+```bash
+python tools/create_data.py sunrgbd --root-path ./data/sunrgbd  --out-dir ./data/sunrgbd --extra-tag sunrgbd
+```
+
 The directory structure after pre-processing should be as below
 ```
 sunrgbd

mmdet3d/datasets/sunrgbd_dataset.py

@@ -48,7 +48,7 @@ class SUNRGBDDataset(Custom3DDataset):
                  ann_file,
                  pipeline=None,
                  classes=None,
-                 modality=None,
+                 modality=dict(use_camera=True, use_lidar=True),
                  box_type_3d='Depth',
                  filter_empty_gt=True,
                  test_mode=False):
@@ -61,8 +61,8 @@ class SUNRGBDDataset(Custom3DDataset):
             box_type_3d=box_type_3d,
             filter_empty_gt=filter_empty_gt,
             test_mode=test_mode)
-        if self.modality is None:
-            self.modality = dict(use_camera=True, use_lidar=True)
+        assert 'use_camera' in self.modality and \
+            'use_lidar' in self.modality
         assert self.modality['use_camera'] or self.modality['use_lidar']
 
     def get_data_info(self, index):
@@ -94,8 +94,9 @@ class SUNRGBDDataset(Custom3DDataset):
             input_dict['file_name'] = pts_filename
 
         if self.modality['use_camera']:
-            img_filename = osp.join(self.data_root,
-                                    info['image']['image_path'])
+            img_filename = osp.join(
+                osp.join(self.data_root, 'sunrgbd_trainval'),
+                info['image']['image_path'])
             input_dict['img_prefix'] = None
             input_dict['img_info'] = dict(filename=img_filename)
             calib = info['calib']