Add to ONNX docs (#13048)

* Add to ONNX docs

* Add MBART example

* Update docs/source/serialization.rst
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

docs/source/serialization.rst

@@ -99,6 +99,30 @@ It will be exported under ``onnx/bert-base-cased``. You should see similar logs:
                     -[✓] all values close (atol: 0.0001)
     All good, model saved at: onnx/bert-base-cased/model.onnx
 
+This export can now be used in the ONNX inference runtime:
+
+.. code-block::
+
+    import onnxruntime as ort
+
+    from transformers import BertTokenizerFast
+    tokenizer = BertTokenizerFast.from_pretrained("bert-base-cased")
+
+    ort_session = ort.InferenceSession("onnx/bert-base-cased/model.onnx")
+
+    inputs = tokenizer("Using BERT in ONNX!", return_tensors="np")
+    outputs = ort_session.run(["last_hidden_state", "pooler_output"], dict(inputs))
+
+The outputs used (:obj:`["last_hidden_state", "pooler_output"]`) can be obtained by taking a look at the ONNX
+configuration of each model. For example, for BERT:
+
+.. code-block::
+
+    from transformers.models.bert import BertOnnxConfig, BertConfig
+
+    config = BertConfig()
+    onnx_config = BertOnnxConfig(config)
+    output_keys = list(onnx_config.outputs.keys())
 
 Implementing a custom configuration for an unsupported architecture
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -142,6 +166,12 @@ An important fact to notice is the use of `OrderedDict` in both inputs and outpu
 as inputs are matched against their relative position within the `PreTrainedModel.forward()` prototype and outputs are
 match against there position in the returned `BaseModelOutputX` instance.
 
+An example of such an addition is visible here, for the MBart model: `Making MBART ONNX-convertible
+<https://github.com/huggingface/transformers/pull/13049/commits/d097adcebd89a520f04352eb215a85916934204f>`__
+
+If you would like to contribute your addition to the library, we recommend you implement tests. An example of such
+tests is visible here: `Adding tests to the MBART ONNX conversion
+<https://github.com/huggingface/transformers/pull/13049/commits/5d642f65abf45ceeb72bd855ca7bfe2506a58e6a>`__
 
 Graph conversion
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