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

doc/api.rst

@@ -53,6 +53,10 @@ Direct data-access through iterators.
    ptbench.data.indian
    ptbench.data.nih_cxr14_re
    ptbench.data.padchest_RS
+   ptbench.data.tbx11k_simplified
+   ptbench.data.tbx11k_simplified_RS
+   ptbench.data.tbx11k_simplified_v2
+   ptbench.data.tbx11k_simplified_v2_RS
 
 
 .. _ptbench.api.models: