[AUTOTVM][Bugfix] Fix history loader for heterogeneous execution

python/tvm/autotvm/task/dispatcher.py

@@ -294,6 +294,7 @@ class ApplyHistoryBest(DispatchContext):
             # use model as key to build best map
             key = (inp.target.model, inp.task.workload)
             if key not in best_by_model:
+                if inp.target.model != 'unknown':
                     best_by_model[key] = (inp, res)
             else:
                 _, other_res = best_by_model[key]