[Swin2SR] Add doc tests
# What does this PR do?

This PR adds Swin2SR to the doc tests.
_The documentation is not available anymore as the PR was closed or merged._<|||||>Yeah I remember a discussion with @patrickvonplaten and @sanchit-gandhi, I used `Swin2SRImageProcessor` here instead of the Auto class to make it more explicit. 

But happy to change. For me, the Auto API is handy when a model doesn't have its own preprocessing class, and for usage in the pipelines.<|||||>Yes I'm with @LysandreJik here! The conclusion was to move towards `AutoProcessor`/`AutoTokenizer` in the docs (_c.f._ the compelling argument from @LysandreJik https://huggingface.slack.com/archives/C01N44FJDHT/p1667824904971239?thread_ts=1667816128.702279&cid=C01N44FJDHT).