Refactor HPO tuner code hierarchy (#3187)

Co-authored-by: liuzhe <liuzhe.cs@pku.edu.cn>

test/ut/sdk/test_hyperopt_tuner.py

@@ -9,7 +9,7 @@ from unittest import TestCase, main
 
 import hyperopt as hp
 
-from nni.algorithms.hpo.hyperopt_tuner.hyperopt_tuner import json2space, json2parameter, json2vals, HyperoptTuner
+from nni.algorithms.hpo.hyperopt_tuner import json2space, json2parameter, json2vals, HyperoptTuner
 
 
 class HyperoptTunerTestCase(TestCase):

test/ut/sdk/test_networkmorphism_tuner.py

@@ -6,6 +6,7 @@ from unittest import TestCase, main
 from copy import deepcopy
 import torch
 
+from nni.algorithms.hpo.networkmorphism_tuner import NetworkMorphismTuner
 from nni.algorithms.hpo.networkmorphism_tuner.graph import graph_to_json, json_to_graph
 from nni.algorithms.hpo.networkmorphism_tuner.graph_transformer import (
     to_deeper_graph,
@@ -13,7 +14,6 @@ from nni.algorithms.hpo.networkmorphism_tuner.graph_transformer import (
     to_wider_graph,
 )
 from nni.algorithms.hpo.networkmorphism_tuner.layers import layer_description_extractor
-from nni.algorithms.hpo.networkmorphism_tuner.networkmorphism_tuner import NetworkMorphismTuner
 from nni.algorithms.hpo.networkmorphism_tuner.nn import CnnGenerator