Fix optional task name.

Co-authored-by: Junru Shao <[email protected]>
Co-authored-by: Bohan Hou <[email protected]>
Co-authored-by: Ruihang Lai <[email protected]>
Co-authored-by: Hongyi Jin <[email protected]>
Co-authored-by: Wuwei Lin <[email protected]>
Co-authored-by: Siyuan Feng <[email protected]>

Author: 7 people

include/tvm/meta_schedule/tune_context.h

@@ -53,7 +53,7 @@ class TuneContextNode : public runtime::Object {
   /*! \brief The probability of using certain mutator. */
   Map<Mutator, FloatImm> mutator_probs;
   /*! \brief The name of the tuning task. */
-  String task_name;
+  Optional<String> task_name;
   /*! \brief The random state. */
   support::LinearCongruentialEngine::TRandState rand_state;
   /*! \brief The number of threads to be used. */