Use new optuna callback mechanism

[hbredin]

Here are a few things the callback function might do:

• every time a better set of hyper-parameters is found, run a validation experiment (possibly asynchronously)
• log trial details to tensorboard (e.g. using tensorboardX)
• display personalized progress report to the user (instead of optuna's builtin logging)

Originally posted by @hbredin in optuna/optuna#302 (comment)