Fix list syncronization with partly empty lists

CHANGELOG.md

@@ -36,6 +36,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Fixed axis names with Precision-Recall curve ([#2462](https://github.com/Lightning-AI/torchmetrics/pull/2462))
 
 
+- Fixed list synchronization with partly empty lists ([#2468](https://github.com/Lightning-AI/torchmetrics/pull/2468))
+
+
 ## [1.3.2] - 2024-03-18
 
 ### Fixed

src/torchmetrics/metric.py

@@ -432,6 +432,10 @@ def _sync_dist(self, dist_sync_fn: Callable = gather_all_tensors, process_group:
             if reduction_fn == dim_zero_cat and isinstance(input_dict[attr], list) and len(input_dict[attr]) > 1:
                 input_dict[attr] = [dim_zero_cat(input_dict[attr])]
 
+            # cornor case in distributed settings where a rank have not received any data, create empty to concatenate
+            if reduction_fn == dim_zero_cat and isinstance(input_dict[attr], list) and len(input_dict[attr]) == 0:
+                input_dict[attr] = [torch.tensor([], device=self.device, dtype=self.dtype)]
+
         output_dict = apply_to_collection(
             input_dict,
             Tensor,

tests/unittests/bases/test_ddp.py

@@ -277,3 +277,18 @@ def _test_sync_with_empty_lists(rank):
 def test_sync_with_empty_lists():
     """Test that synchronization of states can be enabled and disabled for compute."""
     pytest.pool.map(_test_sync_with_empty_lists, range(NUM_PROCESSES))
+
+
+def _test_sync_with_unequal_size_lists(rank):
+    """Test that synchronization of list states work even when some ranks have not received any data yet."""
+    dummy = DummyListMetric()
+    if rank == 0:
+        dummy.update(torch.zeros(2))
+    assert dummy.compute() == tensor([0.0, 0.0])
+
+
+@pytest.mark.DDP()
+@pytest.mark.skipif(sys.platform == "win32", reason="DDP not available on windows")
+def test_sync_with_unequal_size_lists():
+    """Test that synchronization of states can be enabled and disabled for compute."""
+    pytest.pool.map(_test_sync_with_unequal_size_lists, range(NUM_PROCESSES))