Makes the evaluation_function could receive the incumbent best result as input in Tune (#339)

* 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>