Remove imports and use forward references in ONNX feature

src/transformers/onnx/features.py

@@ -1,13 +1,17 @@
 from functools import partial, reduce
-from typing import Callable, Dict, Optional, Tuple, Type, Union
+from typing import TYPE_CHECKING, Callable, Dict, Optional, Tuple, Type, Union
 
 import transformers
 
-from .. import PretrainedConfig, PreTrainedModel, TFPreTrainedModel, is_tf_available, is_torch_available
+from .. import PretrainedConfig, is_tf_available, is_torch_available
 from ..utils import logging
 from .config import OnnxConfig
 
 
+if TYPE_CHECKING:
+    from transformers import PreTrainedModel, TFPreTrainedModel
+
+
 logger = logging.get_logger(__name__)  # pylint: disable=invalid-name
 
 if is_torch_available():
@@ -505,7 +509,7 @@ def get_model_class_for_feature(feature: str, framework: str = "pt") -> Type:
     @staticmethod
     def get_model_from_feature(
         feature: str, model: str, framework: str = "pt", cache_dir: str = None
-    ) -> Union[PreTrainedModel, TFPreTrainedModel]:
+    ) -> Union["PreTrainedModel", "TFPreTrainedModel"]:
         """
         Attempts to retrieve a model from a model's name and the feature to be enabled.
 
@@ -533,7 +537,7 @@ def get_model_from_feature(
 
     @staticmethod
     def check_supported_model_or_raise(
-        model: Union[PreTrainedModel, TFPreTrainedModel], feature: str = "default"
+        model: Union["PreTrainedModel", "TFPreTrainedModel"], feature: str = "default"
     ) -> Tuple[str, Callable]:
         """
         Check whether or not the model has the requested features.