[AutoTVM-FIX] avoid unexpected value(1) of search space when get leng…

…th for uninitiated search space (apache#7175)

* [AutoTVM-FIX] avoid unexpected value(1) of search space when get length for uninitiated search space

* Update python/tvm/autotvm/task/space.py

Co-authored-by: Cody Yu <comaniac0422@gmail.com>

Co-authored-by: ZhaoYanjie <roger.zhao@montage-tech.com>
Co-authored-by: Cody Yu <comaniac0422@gmail.com>

python/tvm/autotvm/task/space.py

@@ -836,6 +836,8 @@ def _add_new_transform(self, space_class, name, axes, policy, **kwargs):
         return [Axis(None, i) for i in range(space_class.get_num_output(axes, policy, **kwargs))]
 
     def __len__(self):
+        if not self.space_map:
+            return 0
         if self._length is None:
             self._length = int(np.prod([len(x) for x in self.space_map.values()]))
         return self._length