* update tune function
* pass incumbent result to the training function
* Update test/tune/test_record_incumbent.py
* Update flaml/searcher/search_thread.py
* Update flaml/searcher/blendsearch.py
* Update flaml/tune/tune.py
* add constant variable
Co-authored-by: 张少坤 <zhangshaokun@fuzhi.ai>
Co-authored-by: Chi Wang <wang.chi@microsoft.com>