[BT] add `BetterTransformer` support for ProphetNet (#923)
* Add ProphetNetEncoder better transformer layer
* add prophetnet to tests
* fix deprecated MutableMapping import
* set original_shape attr to hidden_states
Set original_shape attr once instead of recomputing it every time from
attention mask
* add prophetnet entry to overview
* change test prophetnet model to `hirotasoshu/tiny-random-prophetnet`
