Better logging for tune

[sonichi]

Ideas to improve the logging capabilities in flaml.tune:

• [x] Take a log_file_name argument in tune.run(). Right now it takes local_dir as the directory for the log file, but the log file name is auto generated and can't be overridden.
• [x] When using ray, log the results after the run finishes when log_file_name is given.
• [ ] Provide utility functions to load the log and plot the optimization curve.