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.
 
@@ -206,7 +206,11 @@ 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):
+                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")),