add doc for backend

mmcls/datasets/pipelines/transforms.py

@@ -140,6 +140,8 @@ class RandomResizedCrop(object):
         interpolation (str): Interpolation method, accepted values are
             'nearest', 'bilinear', 'bicubic', 'area', 'lanczos'. Default:
             'bilinear'.
+        backend (str): The image resize backend type, accpeted values are
+            `cv2` and `pillow`. Default: `cv2`.
     """
 
     def __init__(self,
@@ -339,6 +341,8 @@ class Resize(object):
         interpolation (str): Interpolation method, accepted values are
             "nearest", "bilinear", "bicubic", "area", "lanczos".
             More details can be found in `mmcv.image.geometric`.
+        backend (str): The image resize backend type, accpeted values are
+            `cv2` and `pillow`. Default: `cv2`.
     """
 
     def __init__(self, size, interpolation='bilinear', backend='cv2'):