fix(mge/quant): call is_distributed func correctly

GitOrigin-RevId: d8ef0d62ba

imperative/python/megengine/quantization/observer.py

@@ -201,7 +201,7 @@ class SyncExponentialMovingAverageObserver(ExponentialMovingAverageObserver):
     def forward(self, x_orig):
         if self.enabled:
             x = x_orig.detach()
-            if is_distributed:
+            if is_distributed():
                 min_x = all_reduce_min(x.min(), WORLD)
                 max_x = all_reduce_max(x.max(), WORLD)
             else: