Add check if the logger is set in dqn_trainer

Summary: In the base LightningModule class we define an optional logger property which may be initialized to None, while in DQNtrainer._log_dqn method we try to access the logger object without checking first if it was initialized. The issue surfaced when trying to run unit tests analogous to those in test_qrdqn. This commit adds a check whether the logger is initialized prior to attempting to use it. Interestingly, the analogous QRDQNTrainer class implementation does not use the logger property for logging, perhaps it's redundant?

Differential Revision: D37529027

facebookresearch / ReAgent

Add check if the logger is set in dqn_trainer #650