delete useless Value to prevent memory leak (onnx#3760)

Signed-off-by: Chun-Wei Chen <jacky82226@gmail.com>

Co-authored-by: G. Ramalingam <grama@microsoft.com>

Author: jcwchen

onnx/common/ir.h

@@ -1212,6 +1212,7 @@ friend struct Value;
   void freeValue(Value * v) {
     auto it = all_values.find(v);
     ONNX_ASSERT(it != all_values.end());
+    delete *it;
     all_values.erase(it);
   }
 };