Add a new API for tuner "get_best_parameter"

[xuehui1991]

What would you like to be added:

Why is this needed:

Without this feature, how does current nni work：

Components that may involve changes:

Brief description of your proposal if any:

[scarlett2018]

Assign to @xuehui1991 for more information.

[scarlett2018]

@xuehui1991 - is the best parameter from all the completed trials? or to get a "best" parameter predication based on all the info you current have, it is a predication. if it is the later case, it seems similar to getNextParameter().

cc @chicm-ms @QuanluZhang

[savan77]

Hi @scarlett2018 and @xuehui1991 is there any updates on this? I am running hyper parameter optimization trials, but once they are completed I want to know which trial had the highest accuracy/lowest loss?
Is there a way to get this? If you can provide some guidance around this I can also send a PR. Thanks