[Seq2SeqTrainer] Move import to init to make file self-contained
# What does this PR do?

Seq2SeqTrainer can be used as an independent file if no `label_smoothing` is done. This PR move the import to the init to make it possible to simple download this file and use it as it is without any dependencies for standard seq2seq training.

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Would be awesome if @patil-suraj  and @stas00 could review as well :-) 


I'm okay with this, but users will still need to copy paste or rewrite  `Seq2SeqTrainingArguments`   and pass those as `args` instead of default `TrainingArguments`, since we assume `args` is `Seq2SeqTrainingArguments` in multiple methods 

