Added XLM onnx config (#17030)

* Add onnx configuration for xlm

* Add supported features for xlm

* Add xlm to models exportable with onnx

* Add xlm architecture to test file

* Modify docs

* Make code quality fixes

tests/onnx/test_onnx_v2.py

@@ -181,6 +181,7 @@ PYTORCH_EXPORT_MODELS = {
     ("roberta", "roberta-base"),
     ("roformer", "junnyu/roformer_chinese_base"),
     ("mobilebert", "google/mobilebert-uncased"),
+    ("xlm", "xlm-clm-ende-1024"),
     ("xlm-roberta", "xlm-roberta-base"),
     ("layoutlm", "microsoft/layoutlm-base-uncased"),
     ("vit", "google/vit-base-patch16-224"),