restored the world_size == 1 branch so single-process uses to avoid the timeout in CI

Author: littlebullGit

src/lightning/pytorch/callbacks/model_checkpoint.py

@@ -999,6 +999,10 @@ def to_yaml(self, filepath: Optional[_PATH] = None) -> None:
     def file_exists(self, filepath: _PATH, trainer: "pl.Trainer") -> bool:
         """Checks if a file exists on rank 0 and broadcasts the result to all other ranks, preventing the internal
         state to diverge between ranks."""
+        # Single-process or strategies without distributed world size: no need for coordination
+        if trainer.world_size == 1:
+            return self._fs.exists(filepath)
+
         # In distributed setups, only global rank 0 touches the filesystem
         local_decision = self._fs.exists(filepath) if trainer.is_global_zero else False
         # Reduce the decision across ranks using an "any"-style reduction to decide if the file exists anywhere