[C10d] Add skeleton of LibUV backend. (#105672)

This commit hooks up tcpstore creation and build flags.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/105672
Approved by: https://github.com/fduwjj

torch/csrc/distributed/c10d/TCPStore.hpp

@@ -37,6 +37,9 @@ struct TCPStoreOptions {
   // over the bound socket associated to this fd. This option is useful to avoid
   // port assignment races in certain scenarios.
   c10::optional<int> masterListenFd = c10::nullopt;
+
+  // A boolean value indicating whether to use the experimental libUV backend.
+  bool useLibUV = false;
 };
 
 class TORCH_API TCPStore : public Store {