Fix "dim_zero_cat" reduction Lightning-AI#2225

Fixes Lightning-AI#2225 by changing "reduced = global_state + local_state" to "reduced = torch.cat([global_state, local_state]

src/torchmetrics/metric.py

@@ -409,7 +409,7 @@ 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
+                reduced = torch.cat([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):