* immutability fix for seq2seq as well as immutability tests for the collators
* ensure we don't act on none labels and formatting
* remove tf/pt in respective tests as they are not required
* more type error fixes tf/np
* remove todo
* apply suggestions from code review
* formatting / style
* fix seq2seq data collator to respect the given padding strategy
further added tests for the seq2seq data collator in the style of the `data_collator_for_token_classification` (pt, tf, np)
* formatting and change bool equals "==" to "is"
* add missed return types in tests
* update numpy test as it can handle unequal shapes, not like pt or tf