Add Doc Tests for Reformer PyTorch (#16565)
* start working
* fix: ReformerForQA doctest
* fix: ReformerModelWithLMHead doctest
* fix: ReformerModelForSC doctest
* fix: ReformerModelForMLM doctest
* add: documentation_tests.txt
* make fixup
* change: ReformerModelForSC doctest
* change: checkpoint
