[MT5] Fix CONFIG_MAPPING issue leading it to load umt5 class (#24678)

* update

* add umt5 to auto tokenizer mapping

* nits

* fixup

* fix failing torch test

docs/source/en/model_doc/umt5.md

@@ -73,6 +73,9 @@ The conversion script is also different because the model was saved in t5x's lat
 ['<pad><extra_id_0>nyone who<extra_id_1> drink<extra_id_2> a<extra_id_3> alcohol<extra_id_4> A<extra_id_5> A. This<extra_id_6> I<extra_id_7><extra_id_52><extra_id_53></s>']
 ```
 
+## UMT5Config
+
+[[autodoc]] UMT5Config
 
 ## UMT5Model