Fix a FIFO correctness bug (#549)

* Add a FIFO test code that reproduced a correctness issue
* Fix the correctness issue by using pinned memory instead of cudaMemcpy

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Co-authored-by: Binyang Li <binyli@microsoft.com>

python/test/_cpp/proxy_test.cpp

@@ -51,7 +51,7 @@ class MyProxyService {
       semaphores_[nghr]->signal();
       connections_[nghr]->flush();
     }
-    return mscclpp::ProxyHandlerResult::FlushFifoTailAndContinue;
+    return mscclpp::ProxyHandlerResult::Continue;
   }
 
   void start() { proxy_.start(); }