fix a bug in multi_precision_fp16 unittest. (#29756)

python/paddle/fluid/contrib/tests/test_multi_precision_fp16_train.py

@@ -155,9 +155,10 @@ def train(use_pure_fp16=True, use_nesterov=False):
                 loss, = exe.run(compiled_program,
                                 feed=feeder.feed(data),
                                 fetch_list=[sum_cost])
+                loss_v = loss[0] if isinstance(loss, np.ndarray) else loss
                 print('PassID {0:1}, Train Batch ID {1:04}, train loss {2:2.4}'.
-                      format(pass_id, batch_id + 1, float(loss)))
-                train_loss_list.append(float(loss))
+                      format(pass_id, batch_id + 1, float(loss_v)))
+                train_loss_list.append(float(loss_v))
 
                 if batch_id >= 4:  # For speeding up CI
                     test_loss_list = []