Minor fixes to `Trainer` and `BaseTrainer` Initializers

[itsnamgyu]

• self.metric_ftns was previously assigned in BaseTrainer.__init__, but there are no uses of self.metric_ftns in BaseTrainer. What's more, it seems that tracking metrics is not a responsibility of BaseTrainer. Thus, this has been removed from BaseTrainer and self.metric_ftns is now assigned in Trainer.__init__.
• self.config was previously assigned in both BaseTrainer.__init__ and Trainer.__init__. It has been removed from Trainer.