[Fix] Fix multi-task-head loss potential bug (#1530)

* fix bug

* add comments

mmpretrain/models/heads/multi_task_head.py

@@ -22,7 +22,10 @@ def loss_convertor(loss_func, task_name):
                 task_data_samples.append(data_sample.get(task_name))
 
         if len(task_data_samples) == 0:
-            return {'loss': torch.tensor(0.), 'mask_size': torch.tensor(0.)}
+            # This makes it possible to perform loss.backward when a
+            # task does not have gt_labels within a batch.
+            loss = (inputs[0] * 0).sum()
+            return {'loss': loss, 'mask_size': torch.tensor(0.)}
 
         # Mask the inputs of the task
         def mask_inputs(inputs, mask):