[fsmt] deal with -100 indices in decoder ids
Fixes: https://github.com/huggingface/transformers/issues/17945

decoder input ids get the default index -100, which breaks the model - like t5 and many other models add a hardcoded fix to replace -100 with the correct pad index. 

For some reason this use case hasn't been used with this model until recently - so this issue was there since the beginning it seems.

Any suggestions to how to add a simple test here? or perhaps we have something similar already? user's script is quite massive. I think it's Trainer's collate that leads to that padding since it uses -100.

@sgugger 
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