Changed the class on which `register_for_auto_class` method is defined from `TFSequenceSummary` to `TFPreTrainedModel`
Author: azonti
Pull request: yes

# What does this PR do?

Changed the class on which `register_for_auto_class` method is defined from `TFSequenceSummary` to `TFPreTrainedModel`.

It does not make sense that `register_for_auto_class` is defined on `TFSequenceSummary`. I believe this is a bug in PR #15379.

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Comments:
_The documentation is not available anymore as the PR was closed or merged._

Hi @LysandreJik, could you review please? 

Let me ping @sgugger for review, he's more acquainted with this code and will be back from leave shortly :)