[Docs] add docstring for ResNet and ResNeXt (#252)

mmselfsup/models/backbones/resnet.py

@@ -128,7 +128,13 @@ def __init__(self,
         assert max(out_indices) < num_stages + 1
 
     def forward(self, x):
-        """Forward function."""
+        """Forward function.
+
+        As the behavior of forward function in MMSelfSup is different from
+        MMCls, we rewrite the forward function. MMCls does not output the
+        feature map from the 'stem' layer, which we will use for downstream
+        evaluation.
+        """
         if self.deep_stem:
             x = self.stem(x)
         else:

mmselfsup/models/backbones/resnext.py

@@ -13,6 +13,10 @@ class ResNeXt(ResNet):
     Please refer to the `paper <https://arxiv.org/abs/1611.05431>`__ for
     details.
 
+    As the behavior of forward function in MMSelfSup is different from
+    MMCls, we register our own ResNeXt, inheriting from
+    `mmselfsup.model.backbone.ResNet`.
+
     Args:
         depth (int): Network depth, from {50, 101, 152}.
         groups (int): Groups of conv2 in Bottleneck. Defaults to 32.