Add doctests to Perceiver examples (#19129)

* Fix bug in example and add to tests

* Fix failing tests

* Check the size of logits

* Code style

* Try again...

* Add expected loss for PerceiverForMaskedLM doctest

Co-authored-by: Steven Anton <antonstv@amazon.com>
Co-authored-by: ydshieh <ydshieh@users.noreply.github.com>

utils/documentation_tests.txt

@@ -58,6 +58,7 @@ src/transformers/models/opt/modeling_opt.py
 src/transformers/models/opt/modeling_tf_opt.py
 src/transformers/models/owlvit/modeling_owlvit.py
 src/transformers/models/pegasus/modeling_pegasus.py
+src/transformers/models/perceiver/modeling_perceiver.py
 src/transformers/models/plbart/modeling_plbart.py
 src/transformers/models/poolformer/modeling_poolformer.py
 src/transformers/models/reformer/modeling_reformer.py