Owlvit docs test (#18257)

* fix docs and add owlvit docs test

* fix minor bug in post_process, add to processor

* improve owlvit code examples

* fix hardcoded image size

utils/documentation_tests.txt

@@ -48,6 +48,7 @@ src/transformers/models/mobilevit/modeling_mobilevit.py
 src/transformers/models/opt/modeling_opt.py
 src/transformers/models/opt/modeling_tf_opt.py
 src/transformers/models/opt/modeling_flax_opt.py
+src/transformers/models/owlvit/modeling_owlvit.py
 src/transformers/models/pegasus/modeling_pegasus.py
 src/transformers/models/plbart/modeling_plbart.py
 src/transformers/models/poolformer/modeling_poolformer.py