:bug: Handle empty gen_kwargs for seq2seq trainer prediction_step function
# What does this PR do?

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All Trainers expose function `prediction_step`. For `Seq2SeqTrainer`, it seems like the `prediction_step` function is relying on availability of `_gen_kwargs` attribute, which will get set automatically, if `prediction_step` will get called from other functions like `predict` or `evaluate`. However, if someone calls `prediction_step` directly, then this field will not get set and currently will throw `AttributeError: 'Seq2SeqTrainer' object has no attribute '_gen_kwargs'`. 

In this PR, I am trying to resolve above issue by accepting `gen_kwargs` as an argument to `prediction_step` function, in addition to automatically get it from `self` if it has been set previously while falling back to empty `{}` in case its not set in either of those methods. 


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cc: @sgugger
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