[MAINTAIN] Rename train_loop_fn to exec_loop_fn since this is line line with its purpose

[AsadJeewa]

Please describe what needs to be maintained?

Rename train_loop_fn to exec_loop_fn since this is line line with its purpose

Describe the outcome you'd like

Variable name reflects purpose and avoids confusion since training and execution happen separately

How do we know when maintenance is complete?

Checklist:

• [PR merged into develop with name changes]
• [ ]