[Longformer] Major Refactor (#5219)
* refactor naming
* add small slow test
* refactor
* refactor naming
* rename selected to extra
* big global attention refactor
* make style
* refactor naming
* save intermed
* refactor functions
* finish function refactor
* fix tests
* fix longformer
* fix longformer
* fix longformer
* fix all tests but one
* finish longformer
* address sams and izs comments
* fix transpose
