[Enhancement]: use len(result) as batch size (#987)

* use len(result) as batch size

* remove old comments

* Add comments to this modification

* Update test.py

mmcv/engine/test.py

@@ -32,8 +32,8 @@ def single_gpu_test(model, data_loader):
             result = model(return_loss=False, **data)
         results.extend(result)
 
-        # use the first key as main key to calculate the batch size
-        batch_size = len(next(iter(data.values())))
+        # Assume result has the same length of batch_size, refer to https://github.com/open-mmlab/mmcv/issues/985
+        batch_size = len(result)
         for _ in range(batch_size):
             prog_bar.update()
     return results