Add Tensorflow handling of ONNX conversion (#13831)

* Add TensorFlow support for ONNX export

* Change documentation to mention conversion with Tensorflow

* Refactor export into export_pytorch and export_tensorflow

* Check model's type instead of framework installation to choose between TF and Pytorch

Co-authored-by: Lysandre Debut <lysandre@huggingface.co>
Co-authored-by: Alberto Bégué <alberto.begue@della.ai>
Co-authored-by: lewtun <lewis.c.tunstall@gmail.com>

utils/check_table.py

@@ -211,7 +211,7 @@ def check_onnx_model_list(overwrite=False):
     current_list, start_index, end_index, lines = _find_text_in_file(
         filename=os.path.join(PATH_TO_DOCS, "serialization.mdx"),
         start_prompt="<!--This table is automatically generated by `make fix-copies`, do not fill manually!-->",
-        end_prompt="The ONNX conversion is supported for the PyTorch versions of the models.",
+        end_prompt="In the next two sections, we'll show you how to:",
     )
     new_list = get_onnx_model_list()