Update on "Use std::variant to implement pytree Key"

Key was a struct that should've been a union; std::variant makes using a union much easier.

Differential Revision: [D65575184](https://our.internmc.facebook.com/intern/diff/D65575184/)

[ghstack-poisoned]

Author: swolchok

extension/pytree/pytree.h

@@ -55,6 +55,7 @@ using KeyInt = int32_t;
 
 struct Key {
   enum class Kind : uint8_t { None, Int, Str } kind_;
+
  private:
   std::variant<std::monostate, KeyInt, KeyStr> repr_;