[feat] Add sync_context and sync to nn.Metric

torchmetrics/metric.py

@@ -421,6 +421,7 @@ def state_dict(self, destination=None, prefix="", keep_vars=False):
                             current_val = [
                                 cur_v.detach() if isinstance(cur_v, torch.Tensor) else cur_v for cur_v in current_val
                             ]
+                    # the tensors will be synced across processes so deepcopy to drop the references
                     destination[prefix + key] = deepcopy(current_val)
             return destination