[WIP] Introduce Optuna related logics to allennlp.commands.train for supporting pruning in distributed situation

[himkt]

🚧 Under construction 🚧

Related to #5338. See also optuna/optuna#2796.
Other backrgound information can be seen in optuna/optuna#1990 and himkt/allennlp-optuna#20.

Changes proposed in this pull request:

• Introduce trial as an optional argument for train_model and _train_worker

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Reference

• https://guide.allennlp.org/hyperparameter-optimization for understanding the basic architecture of Optuna integration
  • I think it would help to understand how to prune a AllenNLP training procedure in Optuna

[himkt]

I close this PR, as noted in #5338 (comment).