implement download checkpoint

[wanghan-iapcm]

• mark the already-downloaded file, so they will not be download again.
• dflow support skip_exists, but it requires that all artifacts are not tared, which may not be favorable.

[codecov-commenter]

Codecov Report

Base: 81.05% // Head: 81.08% // Increases project coverage by +0.03% 🎉

Coverage data is based on head (a76cc78) compared to base (67c19d6).
Patch coverage: 84.21% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #106      +/-   ##
==========================================
+ Coverage   81.05%   81.08%   +0.03%     
==========================================
  Files          69       69              
  Lines        2887     2903      +16     
==========================================
+ Hits         2340     2354      +14     
- Misses        547      549       +2     

Impacted Files	Coverage Δ
dpgen2/entrypoint/download.py	0.00% <0.00%> (ø)
dpgen2/entrypoint/main.py	0.00% <0.00%> (ø)
dpgen2/entrypoint/watch.py	65.78% <100.00%> (ø)
dpgen2/utils/download_dpgen2_artifacts.py	89.39% <100.00%> (+2.85%)	⬆️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.