Adding support for TFEncoderDecoderModel
Author: thevasudevgupta
Pull request: yes

# What does this PR do?

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This PR will add tf2 support for EncoderDecoderModel upon completion.

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Fixes #9863


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Comments:
Hi @patrickvonplaten,
I just realised that major step will be adding cross-attention layer to `TFDecoderLMHeadModel` for enabling this support. I will start doing that next.

This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

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