Fix typo in quantization callback

Summary: "fb" -> "fn"

Reviewed By: ananthsub

Differential Revision: D29480559

fbshipit-source-id: 78a0cd3ddd25df2c877514d4a5c0c29c248267a2

d2go/runner/callbacks/quantization.py

@@ -433,7 +433,7 @@ class QuantizationAwareTraining(Callback, QuantizationMixin):
             callback.transforms.append(
                 ModelTransform(
                     interval=qat.UPDATE_OBSERVER_STATS_PERIOD,
-                    fb=observer_update_stat,
+                    fn=observer_update_stat,
                     message="Updating observers.",
                 )
             )