Add default cudnn lib path (#25175)

* add default cudnn lib path, test=develop

* change default path in func, test=develop

* move to linux branch, test=develop

* fix var error in other plat, test=develop

paddle/fluid/platform/dynload/dynamic_loader.cc

@@ -58,6 +58,7 @@ struct PathNode {
 };
 
 static constexpr char cupti_lib_path[] = CUPTI_LIB_PATH;
+static constexpr char linux_cudnn_lib_path[] = "/usr/local/cuda/lib64";
 
 static PathNode s_py_site_pkg_path;
 
@@ -200,7 +201,11 @@ void* GetCUDNNDsoHandle() {
 #elif defined(_WIN32) && defined(PADDLE_WITH_CUDA)
   return GetDsoHandleFromSearchPath(FLAGS_cudnn_dir, win_cudnn_lib);
 #else
-  return GetDsoHandleFromSearchPath(FLAGS_cudnn_dir, "libcudnn.so", false);
+  std::string linux_cudnn_path = linux_cudnn_lib_path;
+  if (!FLAGS_cudnn_dir.empty()) {
+    linux_cudnn_path = FLAGS_cudnn_dir;
+  }
+  return GetDsoHandleFromSearchPath(linux_cudnn_path, "libcudnn.so", false);
 #endif
 }