Add Metis Tuner (#534)

* update readme in ga_squad

* update readme

* fix typo

* Update README.md

* Update README.md

* Update README.md

* update readme

* update

* fix path

* update reference

* fix bug in config file

* update nni_arch_overview.png

* update

* update

* update

* add metis tuner code

* 1. fix bug about import 2.update other sdk file

* add auto-gbdt-example and remove unused code

* add metis_tuner into README

* update the README

* update README | remove unused variable

* fix typo

* add sklearn into requirments

* Update src/sdk/pynni/nni/metis_tuner/metis_tuner.py

add default value in __init__
Co-Authored-By: xuehui1991 <xuehui@microsoft.com>

* Update docs/HowToChooseTuner.md
Co-Authored-By: xuehui1991 <xuehui@microsoft.com>

* Update docs/HowToChooseTuner.md
Co-Authored-By: xuehui1991 <xuehui@microsoft.com>

* fix typo | add more comments

tools/nni_cmd/config_schema.py

@@ -68,6 +68,16 @@ Optional('tuner'): Or({
         Optional('n_output_node'):  int,
         },
     Optional('gpuNum'): And(int, lambda x: 0 <= x <= 99999),
+},{
+    'builtinTunerName': 'MetisTuner',
+    'classArgs': {
+        Optional('optimize_mode'): Or('maximize', 'minimize'),
+        Optional('no_resampling'):  bool,
+        Optional('no_candidates'):  bool,
+        Optional('selection_num_starting_points'):  int,
+        Optional('cold_start_num'):  int,
+        },
+    Optional('gpuNum'): And(int, lambda x: 0 <= x <= 99999),
 },{
     'codeDir': os.path.exists,
     'classFileName': str,