MSCCL: Improve executor and integrate scheduler (#694)

* MSCCL: improve executor and add scheduler for testing

* Use external scheduler

* Fix cmake error

* Address comments

* Fix thread safe issue

* Make MSCCL lifecycle APIs thread safe

* Make MSCCL internal scheduler aware of topology hint

* Revise error message

src/include/comm.h

@@ -319,6 +319,9 @@ struct ncclComm {
   bool finalizeCalled;
   // shared structures for finalization
   int finalizeRankCnt;
+
+  // Whether this comm is compatible with MSCCL
+  bool mscclCompatible;
 };
 
 enum ncclLaunchMode {