expose observers to python

Summary: observer framework can now be used in python + a small writeup of how to use it

Reviewed By: salexspb

Differential Revision: D5905002

fbshipit-source-id: e40ec24a55e08fb73beea9b4f3b68e71fc66ffb1

caffe2/python/core.py

@@ -1878,6 +1878,13 @@ class Net(object):
             * [ScopedBlobReference(b) for b in outputs]
         )
 
+    def AddObserver(self, observer):
+        return C.add_observer_to_net(self._net.name, observer)
+
+    # This will return any observer added to the net.
+    def GetObserver(self, index):
+        return C.get_observer_from_net(self._net.name, index)
+
     @property
     def external_inputs(self):
         return [_get_blob_ref(x) for x in self._net.external_input]