[Bug] Fix bug in Collect (#149)

* fix bug in Collect

* add metakeys

mmcls/datasets/pipelines/formating.py

@@ -3,6 +3,7 @@ from collections.abc import Sequence
 import mmcv
 import numpy as np
 import torch
+from mmcv.parallel import DataContainer as DC
 from PIL import Image
 
 from ..builder import PIPELINES
@@ -110,13 +111,34 @@ class Collect(object):
 
     This is usually the last stage of the data loader pipeline. Typically keys
     is set to some subset of "img" and "gt_label".
+
+    Args:
+        keys (Sequence[str]): Keys of results to be collected in ``data``.
+        meta_keys (Sequence[str], optional): Meta keys to be converted to
+            ``mmcv.DataContainer`` and collected in ``data[img_metas]``.
+            Default: ``('filename', 'ori_shape', 'img_shape', 'flip',
+            'flip_direction', 'img_norm_cfg')``
+
+    Returns:
+        dict: The result dict contains the following keys
+                - keys in``self.keys``
+                - ``img_metas`` if avaliable
     """
 
-    def __init__(self, keys):
+    def __init__(self,
+                 keys,
+                 meta_keys=('filename', 'ori_shape', 'img_shape', 'flip',
+                            'flip_direction', 'img_norm_cfg')):
         self.keys = keys
+        self.meta_keys = meta_keys
 
     def __call__(self, results):
         data = {}
+        img_meta = {}
+        for key in self.meta_keys:
+            if key in results:
+                img_meta[key] = results[key]
+        data['img_metas'] = DC(img_meta, cpu_only=True)
         for key in self.keys:
             data[key] = results[key]
         return data