Add get_cat_ids in BaseDataset (#72)

* visualize results on image demo

* add get_cat_ids in BaseDataset

mmcls/datasets/base_dataset.py

@@ -38,12 +38,36 @@ class BaseDataset(Dataset, metaclass=ABCMeta):
 
     @property
     def class_to_idx(self):
+        """Map mapping class name to class index.
+
+        Returns:
+            dict: mapping from class name to class index.
+        """
+
         return {_class: i for i, _class in enumerate(self.CLASSES)}
 
     def get_gt_labels(self):
+        """Get all ground-truth labels (categories).
+
+        Returns:
+            list[int]: categories for all images.
+        """
+
         gt_labels = np.array([data['gt_label'] for data in self.data_infos])
         return gt_labels
 
+    def get_cat_ids(self, idx):
+        """Get category id by index.
+
+        Args:
+            idx (int): Index of data.
+
+        Returns:
+            int: Image category of specified index.
+        """
+
+        return self.data_infos[idx]['gt_label'].astype(np.int)
+
     def prepare_data(self, idx):
         results = copy.deepcopy(self.data_infos[idx])
         return self.pipeline(results)