Moved code to lightning

Updating codebase to make use of pytorch-lightning

• Main train/valid loops are handled by lightning
• Removed device-handling code
• Each model implements its own train/valid/predict steps
• Updated config files for use with lightning
• Using lightning checkpointing and logging callbacks
• Updated DeviceMonitor to be compatible with lightning

src/ptbench/engine/trainer.py

@@ -189,7 +189,7 @@ def run(
         save a checkpoint every ``n`` epochs.  If set to ``0`` (zero), then do
         not save intermediary checkpoints
 
-    accelerator : str`
+    accelerator : str
         accelerator to use
 
     arguments : dict