revert acc changes (#9731)

* Revert "avoid per-batch blocking in metric (#9636)"

This reverts commit 3fe694e.

* Revert "proper flatten in acc (#9619)"

This reverts commit ed823b2.

* Revert "use nd for accuracy calculation (#9583)"

This reverts commit f5f1b91.

* keep doc change

python/mxnet/metric.py

@@ -391,16 +391,13 @@ def update(self, labels, preds):
         for label, pred_label in zip(labels, preds):
             if pred_label.shape != label.shape:
                 pred_label = ndarray.argmax(pred_label, axis=self.axis)
-            pred_label = pred_label.astype('int32')
-            label = label.astype('int32')
+            pred_label = pred_label.asnumpy().astype('int32')
+            label = label.asnumpy().astype('int32')
 
             check_label_shapes(label, pred_label)
 
-            if pred_label.context != label.context:
-                pred_label = pred_label.as_in_context(label.context)
-
-            self.sum_metric += (pred_label.flatten() == label.flatten()).sum().asscalar()
-            self.num_inst += numpy.prod(pred_label.shape)
+            self.sum_metric += (pred_label.flat == label.flat).sum()
+            self.num_inst += len(pred_label.flat)
 
 
 @register