[FuturWarning] Add futur warning for LEDForSequenceClassification
# What does this PR do?
fixes #19019 by replacing the construction of the `eos_mask` in the `SequenceClassification`. 
Also adds a test to make sure that long sequence are properly processed


_The documentation is not available anymore as the PR was closed or merged._<|||||>I think deleting the class would be less confusing, we discussed this with Patrick, the original paper does not use the encoder decoder model for sequence classification. WDYT @LysandreJik <|||||>Let's please do a deprecation cycle :pray: <|||||>Would love to learn about, what can I do? <|||||>I think there is still work to do here?<|||||>@ArthurZucker, instead of deleting the class, you would first start by adding a `FutureWarning` when it is instantiated or called, mentioning that it is deprecated and what is recommended instead. You would mention that the class will be deleted in version 5, and that such a code will error out then.<|||||>Oh it ! Thanks for the pointers 