src/transformers/onnx/features.py/FeaturesManager/_validate_framework_choice
class FeaturesManager:    def _validate_framework_choice(framework: str):
        """
        Validates if the framework requested for the export is both correct and available, otherwise throws an
        exception.
        """
        if framework not in ["pt", "tf"]:
            raise ValueError(
                f"Only two frameworks are supported for ONNX export: pt or tf, but {framework} was provided."
            )
        elif framework == "pt" and not is_torch_available():
            raise RuntimeError("Cannot export model to ONNX using PyTorch because no PyTorch package was found.")
        elif framework == "tf" and not is_tf_available():
            raise RuntimeError("Cannot export model to ONNX using TensorFlow because no TensorFlow package was found.")