fix fstr format (#2810)

* fix fstr format

* Quality pass

src/accelerate/accelerator.py

@@ -3123,7 +3123,9 @@ def _inner(folder):
             f for f in os.listdir(input_dir) if re.search(r"^custom_checkpoint_\d+\.pkl$", f) is not None
         ]
         if len(custom_checkpoints) != len(self._custom_objects):
-            err = "Number of custom checkpoints in folder {input_dir} does not match the number of registered objects:"
+            err = (
+                f"Number of custom checkpoints in folder {input_dir} does not match the number of registered objects:"
+            )
             err += f"\n\tFound checkpoints: {len(custom_checkpoints)}"
             err += f"\n\tRegistered objects: {len(self._custom_objects)}\n"
             err += "Please make sure to only load checkpoints from folders that were created with the same set of registered objects,"

src/accelerate/state.py

@@ -181,7 +181,7 @@ def __init__(self, cpu: bool = False, **kwargs):
             original_backend = kwargs.pop("backend", None)
             backend, distributed_type = self._prepare_backend(cpu, use_sagemaker_dp, original_backend)
             if original_backend is not None and backend != original_backend:
-                raise ValueError("Your assigned backend {original_backend} is not avaliable, please use {backend}")
+                raise ValueError(f"Your assigned backend {original_backend} is not avaliable, please use {backend}")
             self.backend = backend
             self.distributed_type = distributed_type
             use_deepspeed = False