Preserve order while deduplicating in ChoiceParameter

[saitcakmak]

Summary: list(set(some_list)) does not preserve the order of the original list. This both makes the test flaky and may cause issues if used with is_ordered=True. list(dict.fromkeys(some_list)) deduplicates while preserving the original order.

Reviewed By: Balandat

Differential Revision: D53599623