make save dir for each model training job

[dkimpara]

for fsdp: its model_checkpoint.pt
for everything else: checkpoint.pt

[dkimpara]

conclusion: specify checkpoint name under save_loc. add default behavior to parser, change train.py

[dkimpara]

update on renaming checkpoints: i've edited the bits of the codebase i'm familiar with but i'm not familiar the other parts that need to happen:

• need to name model checkpoints we're saving to
• need to name optimizer checkpoints loading from/to

I think it would be less complexity to make a new dir for each training run - based on a timestamp or similar. this also safeguards against accidentally overwriting files.

I've left the progress i've made in the branch rename_checkpoint