[Trainer] refactor/consolidate trainer implementations

[casteryh]

Context

There are code duplications in our implementations of the trainer.
For example, there is torchforge/src/forge/actors/trainer/titan.py and there is a standalone sft trainer implementation in apps/sft/main.py.

To do

• refactor our trainer implementation in torchforge/src/forge/actors/trainer/titan.py to support, for example, SFT
• use the new TitanTrainer in the SFT app