Serialize union fields with single entry dict. (#2279)

Summary:
X-link: https://github.com/pytorch/pytorch/pull/121337

Pull Request resolved: https://github.com/pytorch/executorch/pull/2279

remove "$type" and "$value" fields, instead only serialize as {type: value} for union fields directly.

bypass-github-export-checks

Reviewed By: tugsbayasgalan

Differential Revision: D54600943

fbshipit-source-id: d4bab91541db42eec2d8ead1176db6acf0db8ef3