:rotating_light: [`v5`] Remove headmasking (#41076)
* first attempt at removing
* copies
* last bits in core
* quick fixes
* tests purge
* docs and examples
* some fixes
* more
* another round of cleanups
* fix
* fix a bunch of models
* fix dummy bert
* fix
* fix new model
* fix signature change
* fix
* fix style/copies
* new models
* fix copies didnt find that damn
* test
* this shouldnt have happened during model addition
