Fix segfault in `THPGenerator_dealloc` (#42490)
Summary:
Segfault happens when one tries to deallocate unintialized generator
Add `TestTorch.test_invalid_generator_raises` that validates that Generator created on invalid device is handled correctly
Fixes https://github.com/pytorch/pytorch/issues/42281
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42490
Reviewed By: seemethere
Differential Revision: D22908795
Pulled By: malfet
fbshipit-source-id: c5b6a35db381738c0fc984aa54e5cab5ef2cbb76
