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>
