fix bugs for cropping masks in RandomCrop (#1333)

* add g++ requirement for INSTALL.md.

* fix bugs for cropping masks in RandomCrop.

* simplify the code to fix bugs for cropping masks in RandomCrop.

mmdet/datasets/pipelines/transforms.py

@@ -300,7 +300,7 @@ class Normalize(object):
 
 @PIPELINES.register_module
 class RandomCrop(object):
-    """Random crop the image & bboxes.
+    """Random crop the image & bboxes & masks.
 
     Args:
         crop_size (tuple): Expected size after cropping, (h, w).
@@ -348,7 +348,7 @@ class RandomCrop(object):
             # filter and crop the masks
             if 'gt_masks' in results:
                 valid_gt_masks = []
-                for i in valid_inds:
+                for i in np.where(valid_inds)[0]:
                     gt_mask = results['gt_masks'][i][crop_y1:crop_y2, crop_x1:
                                                      crop_x2]
                     valid_gt_masks.append(gt_mask)