Unconditinally use typing extensions in jit_internal (#42538)
Summary:
Since https://github.com/pytorch/pytorch/issues/38221 is closed now, `typing_extensions` module should always be available
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42538
Reviewed By: ezyang
Differential Revision: D22942153
Pulled By: malfet
fbshipit-source-id: edabbadde13800a3412d14c19ca55ef206ada5e1
