[VisionEncoderDecoder] Add gradient checkpointing
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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@fgbelidji can you check whether this works?


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Models:

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Comments:
_The documentation is not available anymore as the PR was closed or merged._

@patrickvonplaten this works, thanks! 

Tests added - good to merge cc @NielsRogge FYI