Don't throw fatal exception during 'import cudaq' (#1152)

If the user has GPUs but doesn't have all the right dependencies for a
GPU-based simulator, we should not throw fatal exceptions in that case.

python/utils/LinkedLibraryHolder.cpp

@@ -1,5 +1,5 @@
 /*******************************************************************************
- * Copyright (c) 2022 - 2023 NVIDIA Corporation & Affiliates.                  *
+ * Copyright (c) 2022 - 2024 NVIDIA Corporation & Affiliates.                  *
  * All rights reserved.                                                        *
  *                                                                             *
  * This source code and the accompanying materials are made available under    *
@@ -255,10 +255,27 @@ LinkedLibraryHolder::LinkedLibraryHolder() {
 
   // Set the default target
   // If environment variable set with a valid value, use it
-  // Otherwise, if GPU(s) available, set default to 'nvidia', else to 'qpp-cpu'
+  // Otherwise, if GPU(s) available and other dependencies are satisfied, set
+  // default to 'nvidia', else to 'qpp-cpu'
   defaultTarget = "qpp-cpu";
   if (countGPUs() > 0) {
-    defaultTarget = "nvidia";
+    // Before setting the defaultTarget to nvidia, make sure the simulator is
+    // available.
+    const std::string nvidiaTarget = "nvidia";
+    auto iter = targets.find(nvidiaTarget);
+    if (iter != targets.end()) {
+      auto target = iter->second;
+      if (std::find(availableSimulators.begin(), availableSimulators.end(),
+                    target.simulatorName) != availableSimulators.end())
+        defaultTarget = nvidiaTarget;
+      else
+        cudaq::info(
+            "GPU(s) found but cannot select nvidia target because simulator "
+            "is not available. Are all dependencies installed?");
+    } else {
+      cudaq::info("GPU(s) found but cannot select nvidia target because nvidia "
+                  "target not found.");
+    }
   }
   auto env = std::getenv("CUDAQ_DEFAULT_SIMULATOR");
   if (env) {