change the way sentinel tokens can retrived
# What does this PR do?

fixes the issue #19298 

@SaulLu 
_The documentation is not available anymore as the PR was closed or merged._<|||||>@SaulLu  @sgugger  Done<|||||>As pointed above, you're still missing the stronger test to detect the sentinel tokens.<|||||>Thanks a lot for working on this. Maybe I'm just nitpicking, but wouldn't it be great if the returned sentinel tokens were sorted?

```
self.tokenizer.get_sentinel_tokens()
['<extra_id_1>', '<extra_id_26>', '<extra_id_55>', '<extra_id_87>', '<extra_id_50>', '<extra_id_49>', '<extra_id_74>', '<extra_id_66>', '<extra_id_83>', '<extra_id_30>', '<extra_id_3>', '<extra_id_0>', '<extra_id_90>', '<extra_id_14>', '<extra_id_71>', '<extra_id_6>', '<extra_id_18>', '<extra_id_4>', '<extra_id_75>', '<extra_id_99>', '<extra_id_63>', '<extra_id_58>', '<extra_id_48>', '<extra_id_62>', '<extra_id_73>', '<extra_id_20>', '<extra_id_70>', '<extra_id_21>', '<extra_id_38>', '<extra_id_34>', '<extra_id_88>', '<extra_id_28>', '<extra_id_97>', '<extra_id_91>', '<extra_id_65>', '<extra_id_81>', '<extra_id_98>', '<extra_id_23>', '<extra_id_96>', '<extra_id_12>', '<extra_id_19>', '<extra_id_79>', '<extra_id_78>', '<extra_id_68>', '<extra_id_95>', '<extra_id_35>', '<extra_id_42>', '<extra_id_27>', '<extra_id_85>', '<extra_id_67>', '<extra_id_17>', '<extra_id_36>', '<extra_id_93>', '<extra_id_37>', '<extra_id_60>', '<extra_id_77>', '<extra_id_32>', '<extra_id_92>', '<extra_id_33>', '<extra_id_40>', '<extra_id_86>', '<extra_id_53>', '<extra_id_10>', '<extra_id_31>', '<extra_id_72>', '<extra_id_24>', '<extra_id_80>', '<extra_id_13>', '<extra_id_45>', '<extra_id_61>', '<extra_id_52>', '<extra_id_8>', '<extra_id_44>', '<extra_id_57>', '<extra_id_16>', '<extra_id_84>', '<extra_id_51>', '<extra_id_56>', '<extra_id_41>', '<extra_id_64>', '<extra_id_39>', '<extra_id_9>', '<extra_id_25>', '<extra_id_59>', '<extra_id_46>', '<extra_id_7>', '<extra_id_54>', '<extra_id_29>', '<extra_id_89>', '<extra_id_22>', '<extra_id_15>', '<extra_id_43>', '<extra_id_47>', '<extra_id_76>', '<extra_id_5>', '<extra_id_94>', '<extra_id_11>', '<extra_id_82>', '<extra_id_2>', '<extra_id_69>']
```