Conform asidv to the new description api

[jon-tow]

Also ensures the DATASET_PATH directory is created with parents.

[codecov]

Codecov Report

Merging #256 (caff4f1) into master (0fe2e80) will decrease coverage by 0.23%.
The diff coverage is 87.24%.

@@            Coverage Diff             @@
##           master     #256      +/-   ##
==========================================
- Coverage   95.76%   95.52%   -0.24%     
==========================================
  Files          45       46       +1     
  Lines        3731     3756      +25     
==========================================
+ Hits         3573     3588      +15     
- Misses        158      168      +10     

Impacted Files	Coverage Δ
lm_eval/tasks/anli.py	100.00% <ø> (ø)
lm_eval/tasks/arc.py	100.00% <ø> (ø)
lm_eval/tasks/glue.py	96.44% <ø> (+0.24%)	⬆️
lm_eval/tasks/headqa.py	100.00% <ø> (ø)
lm_eval/tasks/hellaswag.py	100.00% <ø> (ø)
lm_eval/tasks/lambada.py	92.85% <ø> (-0.33%)	⬇️
lm_eval/tasks/lambada_cloze.py	100.00% <ø> (ø)
lm_eval/tasks/lambada_multilingual.py	94.00% <ø> (ø)
lm_eval/tasks/mathqa.py	100.00% <ø> (ø)
lm_eval/tasks/mc_taco.py	100.00% <ø> (ø)
... and 36 more

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