cache: cache in dvc exp if not stage output

[dberenbaum]

Per #631 (comment)

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.02% ⚠️

Comparison is base (bdf39da) 88.30% compared to head (e5a462d) 88.28%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #660      +/-   ##
==========================================
- Coverage   88.30%   88.28%   -0.02%     
==========================================
  Files          43       43              
  Lines        3139     3134       -5     
  Branches      279      279              
==========================================
- Hits         2772     2767       -5     
  Misses        325      325              
  Partials       42       42              

Files Changed	Coverage Δ
src/dvclive/live.py	94.62% <100.00%> (-0.02%)	⬇️
tests/test_log_artifact.py	100.00% <100.00%> (ø)

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