Update Readme for save_model

Signed-off-by: Jael Gu <mengjia.gu@zilliz.com>

README.md

@@ -362,6 +362,14 @@ Save model to local with specified format.
 
 ​	The path where model is saved to. By default, it will save model to the operator directory.
 
+```python
+from towhee import ops
+
+op = ops.text_embedding.transformers(model_name='distilbert-base-cased').get_op()
+op.save_model('onnx', 'test.onnx')
+```
+PosixPath('/Home/.towhee/operators/text-embedding/transformers/main/test.onnx')
+
 <br />
 
 ***supported_model_names(format=None)***

auto_transformers.py

@@ -140,14 +140,14 @@ class AutoTransformers(NNOperator):
         dummy_input = '[CLS]'
         inputs = self.tokenizer(dummy_input, return_tensors='pt')  # a dictionary
         if model_type == 'pytorch':
-            torch.save(self.model, output_file)
+            torch.save(self._model, output_file)
         elif model_type == 'torchscript':
             inputs = list(inputs.values())
             try:
                 try:
-                    jit_model = torch.jit.script(self.model)
+                    jit_model = torch.jit.script(self._model)
                 except Exception:
-                    jit_model = torch.jit.trace(self.model, inputs, strict=False)
+                    jit_model = torch.jit.trace(self._model, inputs, strict=False)
                 torch.jit.save(jit_model, output_file)
             except Exception as e:
                 log.error(f'Fail to save as torchscript: {e}.')
@@ -171,7 +171,7 @@ class AutoTransformers(NNOperator):
         # todo: elif format == 'tensorrt':
         else:
             log.error(f'Unsupported format "{format}".')
-        return True
+        return Path(output_file).resolve()
 
     @property
     def supported_formats(self):