[Bugfix, CuDNN] fix segfault when cudnnDestroy called with destroyed cuda context (#8267)

* fix: cudnnDestroy called after cuda context is over

* refact: rename global var with `g_`

* clang-format

* refact: let cudnn handlers leak

Author: ganler

src/runtime/contrib/cudnn/cudnn_utils.cc

@@ -115,11 +115,7 @@ CuDNNThreadEntry::CuDNNThreadEntry() {
   conv_entry.cuda_api = cuda_api;
 }
 
-CuDNNThreadEntry::~CuDNNThreadEntry() {
-  if (handle) {
-    CUDNN_CALL(cudnnDestroy(handle));
-  }
-}
+CuDNNThreadEntry::~CuDNNThreadEntry() {}
 
 typedef dmlc::ThreadLocalStore<CuDNNThreadEntry> CuDNNThreadStore;