Fix dim_zero_cat reduction

CHANGELOG.md

@@ -63,6 +63,9 @@ 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

@@ -409,7 +409,10 @@ 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):