Fix evaluation if device not specified

[awaelchli]

The command we have in the docs fails:

litgpt evaluate \
  --checkpoint_dir checkpoints/microsoft/phi-2/ \
  --batch_size 16 \
  --tasks "hellaswag" \
  --out_dir evaluate_model

Traceback (most recent call last):
  File "/home/zeus/miniconda3/envs/cloudspace/bin/litgpt", line 8, in <module>
    sys.exit(main())
  File "/home/zeus/miniconda3/envs/cloudspace/lib/python3.10/site-packages/litgpt/__main__.py", line 143, in main
    fn(**kwargs)
  File "/home/zeus/miniconda3/envs/cloudspace/lib/python3.10/site-packages/litgpt/eval/evaluate.py", line 99, in convert_and_evaluate
    model = HFLM(repo_id, state_dict=state_dict, device=device, batch_size=batch_size, dtype=dtype)
  File "/home/zeus/miniconda3/envs/cloudspace/lib/python3.10/site-packages/lm_eval/models/huggingface.py", line 143, in __init__
    assert isinstance(device, str)
AssertionError

Because device=None is not accepted in lm_eval even though the type would suggest it is. This could be considered a bug, but we can choose a good default based on device availability.

[rasbt]

Good catch, I should have tested that when I added the code. Thanks for fixing!