Revert "[export] Serialize union fields with single entry dict. (#121263)"
This reverts commit 7feabe9b73e6ba7724b62ea91df27049defdf378.
Reverted https://github.com/pytorch/pytorch/pull/121263 on behalf of https://github.com/osalpekar due to A large number of inductor benchmarking jobs failing starting this PR. See for details: https://hud.pytorch.org/pytorch/pytorch/commit/7feabe9b73e6ba7724b62ea91df27049defdf378 ([comment](https://github.com/pytorch/pytorch/pull/121263#issuecomment-1981680049))
