Summary: So that it is possible to have both `Foo<K0, K1, X, Y>` and `Bar<K0, K1, A, B>` where both `Foo` and `Bar` use `UniqueInstance`.

Differential Revision: D27600228

fbshipit-source-id: a5d58726ea0009d7a45c0234f9cff723662a4604