[composite compliance testing] remove tree_flatten hack (#81057)
Previously we had a hack for tree_flatten not supporting
torch.return_types. That was fixed a while ago
(https://github.com/pytorch/pytorch/issues/74624) so we can delete the
hack.
Test Plan:
- wait for tests
Pull Request resolved: https://github.com/pytorch/pytorch/pull/81057
Approved by: https://github.com/kshitij12345, https://github.com/ezyang
