[tune] change trial dir default to include name

[richardliaw]

Why are these changes needed?

Make the default trial dir also include the trial name.

Related issue number

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[janblumenkamp]

Thanks, Richard! Is the date_str() necessary because there are no guarantees that the trial id is unique?
Maybe it would make sense to somehow unify this across all loggers (I am particularly thinking about the wandb logger, maybe there are others that are affected by this). For example, in the wandb logger, only the string representation of a trial is shown as name. It would be good if the name shown in the wandb logger would be exactly the same as the name of the directory that contains the checkpoints (so str(trial)+date_str() or something like that).

[richardliaw]

Hmm @janblumenkamp - unfortunately the tensorboard visualization sort of requires us to append a nasty string to the end of the directory name. We could perhaps get rid of the timestring if you think that's too verbose. We have it there for convenience (i.e., to see which trials were created when).