Bugfix: fix compute when called on empty lists (#1219)

* fix ##1218
* tests
* changelog

Co-authored-by: Aws user for bootstrap <aws_install@ip-10-102-147-234.ef73-poctrustai.aws.cloud.airbus-v.corp>
Co-authored-by: SkafteNicki <[email protected]>

CHANGELOG.md

@@ -69,6 +69,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   * Jaccard index ([#1205](https://github.com/Lightning-AI/metrics/pull/1205))
 
 
+- Fixed synchronization of empty list states ([#1219](https://github.com/Lightning-AI/metrics/pull/1219))
+
+
 ## [0.9.3] - 2022-08-22
 
 ### Added

src/torchmetrics/metric.py

@@ -371,6 +371,10 @@ def _sync_dist(self, dist_sync_fn: Callable = gather_all_tensors, process_group:
         for attr, reduction_fn in self._reductions.items():
             # pre-processing ops (stack or flatten for inputs)
 
+            if isinstance(output_dict[attr], list) and len(output_dict[attr]) == 0:
+                setattr(self, attr, [])
+                continue
+
             if isinstance(output_dict[attr][0], Tensor):
                 output_dict[attr] = torch.stack(output_dict[attr])
             elif isinstance(output_dict[attr][0], list):

tests/unittests/bases/test_ddp.py

@@ -273,3 +273,16 @@ def _test_sync_on_compute_list_state(rank, worldsize, sync_on_compute):
 def test_sync_on_compute(sync_on_compute, test_func):
     """Test that syncronization of states can be enabled and disabled for compute."""
     torch.multiprocessing.spawn(test_func, args=(2, sync_on_compute), nprocs=2)
+
+
+def _test_sync_with_empty_lists(rank, worldsize):
+    setup_ddp(rank, worldsize)
+    dummy = DummyListMetric()
+    val = dummy.compute()
+    assert val == []
+
+
+@pytest.mark.skipif(sys.platform == "win32", reason="DDP not available on windows")
+def test_sync_with_empty_lists():
+    """Test that syncronization of states can be enabled and disabled for compute."""
+    torch.multiprocessing.spawn(_test_sync_with_empty_lists, args=(2,), nprocs=2)