Fix 3-way merging with the checkpoint_merger community pipeline (#2355)

correctly locate 3rd file; also correct misleading docs

examples/community/checkpoint_merger.py

@@ -80,8 +80,8 @@ class CheckpointMergerPipeline(DiffusionPipeline):
                 alpha - The interpolation parameter. Ranges from 0 to 1.  It affects the ratio in which the checkpoints are merged. A 0.8 alpha
                     would mean that the first model checkpoints would affect the final result far less than an alpha of 0.2
 
-                interp - The interpolation method to use for the merging. Supports "sigmoid", "inv_sigmoid", "add_difference" and None.
-                    Passing None uses the default interpolation which is weighted sum interpolation. For merging three checkpoints, only "add_difference" is supported.
+                interp - The interpolation method to use for the merging. Supports "sigmoid", "inv_sigmoid", "add_diff" and None.
+                    Passing None uses the default interpolation which is weighted sum interpolation. For merging three checkpoints, only "add_diff" is supported.
 
                 force - Whether to ignore mismatch in model_config.json for the current models. Defaults to False.
 
@@ -206,14 +206,18 @@ class CheckpointMergerPipeline(DiffusionPipeline):
                         )
                     )
                     checkpoint_path_1 = files[0] if len(files) > 0 else None
-                if checkpoint_path_2 is not None and os.path.exists(checkpoint_path_2):
-                    files = list(
-                        (
-                            *glob.glob(os.path.join(checkpoint_path_2, "*.safetensors")),
-                            *glob.glob(os.path.join(checkpoint_path_2, "*.bin")),
+                if len(cached_folders) < 3:
+                    checkpoint_path_2 = None
+                else:
+                    checkpoint_path_2 = os.path.join(cached_folders[2], attr)
+                    if os.path.exists(checkpoint_path_2):
+                        files = list(
+                            (
+                                *glob.glob(os.path.join(checkpoint_path_2, "*.safetensors")),
+                                *glob.glob(os.path.join(checkpoint_path_2, "*.bin")),
+                            )
                         )
-                    )
-                    checkpoint_path_2 = files[0] if len(files) > 0 else None
+                        checkpoint_path_2 = files[0] if len(files) > 0 else None
                 # For an attr if both checkpoint_path_1 and 2 are None, ignore.
                 # If atleast one is present, deal with it according to interp method, of course only if the state_dict keys match.
                 if checkpoint_path_1 is None and checkpoint_path_2 is None: