Fix dim_zero_cat reduction (#2226)

Co-authored-by: Nicki Skafte Detlefsen <skaftenicki@gmail.com>
(cherry picked from commit 9652899)

CHANGELOG.md

@@ -41,6 +41,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Fixed device and dtype for `LearnedPerceptualImagePatchSimilarity` functional metric ([#2234](https://github.com/Lightning-AI/torchmetrics/pull/2234))
 
 
+- Fixed bug in `Metric._reduce_states(...)` when using `dist_sync_fn="cat"` ([#2226](https://github.com/Lightning-AI/torchmetrics/pull/2226))
+
+
 ## [1.2.0] - 2023-09-22
 
 ### Added

src/torchmetrics/metric.py

@@ -404,7 +404,10 @@ def _reduce_states(self, incoming_state: Dict[str, Any]) -> None:
             elif reduce_fn == dim_zero_min:
                 reduced = torch.min(global_state, local_state)
             elif reduce_fn == dim_zero_cat:
-                reduced = global_state + local_state
+                if isinstance(global_state, Tensor):
+                    reduced = torch.cat([global_state, local_state])
+                else:
+                    reduced = global_state + local_state
             elif reduce_fn is None and isinstance(global_state, Tensor):
                 reduced = torch.stack([global_state, local_state])
             elif reduce_fn is None and isinstance(global_state, list):