Update convert_from_ckpt.py / read checkpoint config yaml contents (#6633)

Update convert_from_ckpt.py / read config yaml contents

Added missing reading of config yaml file contents

src/diffusers/pipelines/stable_diffusion/convert_from_ckpt.py

@@ -1317,6 +1317,9 @@ def download_from_original_stable_diffusion_ckpt(
 
         if config_url is not None:
             original_config_file = BytesIO(requests.get(config_url).content)
+        else:
+            with open(original_config_file, "r") as f:
+                original_config_file = f.read()
 
     original_config = yaml.safe_load(original_config_file)