Add docs for SomeWeight.DEFAULT fields

docs/source/conf.py

@@ -297,12 +297,33 @@ def inject_minigalleries(app, what, name, obj, options, lines):
 
 
 def inject_weight_metadata(app, what, name, obj, options, lines):
+    """This hook is used to generate docs for the models weights.
+
+    Objects like ResNet18_Weights are enums with fields, where each field is a Weight object.
+    Enums aren't easily documented in Python so the solution we're going for is to:
+
+    - add an autoclass directive in the model's builder docstring, e.g.
+
+    ```
+    .. autoclass:: torchvision.models.ResNet34_Weights
+        :members:
+    ```
+
+    (see resnet.py for an example)
+    - then this hook is called automatically when building the docs, and it generates the text that gets
+      used within the autoclass directive.
+    """
 
     if obj.__name__.endswith("_Weights"):
-        lines[:] = ["The model builder above accepts the following values as the ``weights`` parameter:"]
+        lines[:] = [
+            "The model builder above accepts the following values as the ``weights`` parameter.",
+            f"``{obj.__name__}.DEFAULT`` is equivalent to ``{obj.DEFAULT}``.",
+        ]
         lines.append("")
         for field in obj:
             lines += [f"**{str(field)}**:", ""]
+            if field == obj.DEFAULT:
+                lines += [f"This weight is also available as ``{obj.__name__}.DEFAULT``.", ""]
 
             table = []
             for k, v in field.meta.items():