fix model checkpoint merger tab failing to save model

modules/extras.py

@@ -191,9 +191,11 @@ def run_modelmerger(primary_model_name, secondary_model_name, interp_method, int
             if save_as_half:
                 theta_0[key] = theta_0[key].half()
 
+    ckpt_dir = shared.cmd_opts.ckpt_dir or sd_models.model_path
+
     filename = primary_model_info.model_name + '_' + str(round(interp_amount, 2)) + '-' + secondary_model_info.model_name + '_' + str(round((float(1.0) - interp_amount), 2)) + '-' + interp_method.replace(" ", "_") + '-merged.ckpt'
     filename = filename if custom_name == '' else (custom_name + '.ckpt')
-    output_modelname = os.path.join(shared.cmd_opts.ckpt_dir, filename)
+    output_modelname = os.path.join(ckpt_dir, filename)
 
     print(f"Saving to {output_modelname}...")
     torch.save(primary_model, output_modelname)