"parlai eval_model" doesn't produce tensorboard report when "--tensorboard-log" is enabled

[DrMatters]

Command to reproduce the bug:

parlai eval_model -t blended_skill_talk -mf zoo:blender/blender_90M/model --skip-generation True --tensorboard-log True --tensorboard-logdir ~/tb_report_for_issuue/

A user expects to see a folder called tb_report_for_issuue in the home directory containing the evaluation report data. The expected behavior is the same as when enabling tensorboard logging with parlai train_model.

P.S. It may be helpful to add other loggers (such as WandB) to the eval_model script.

[stephenroller]

Considering TB expects a time series, not a static evaluation, what should be logged?

Happy to accept a patch for this if you've got an idea.

[DrMatters]

PR: #4497
@stephenroller