Refactor training service, add non-running mode

[SparkSnail]

Refactor trainingService, give interface to users, let users call trainingService function directly from nnictl. For example, move delete experiment feature to trainingService, nnictl will call function from trainingService to delete experiment.

[QuanluZhang]

@SparkSnail can we close this issue now?

[SparkSnail]

I don't remember the scenario to use this feature, perhaps we could have a short discussion about this feature, and close it if it's not necessary to be implemented.

[scarlett2018]

I don't remember the scenario to use this feature, perhaps we could have a short discussion about this feature, and close it if it's not necessary to be implemented.

any conclusion from the discussion?