feat: warn when SLURM ranks double count a node

[davidberenstein1957]

Split out of #1366, which mixed it in with the job-metadata columns. It is unrelated scope and stands on its own.

The hazard

On SLURM, srun --ntasks-per-node=4 starts four ranks on one node. With tracking_mode="machine" — the default — each of those four trackers measures the whole node, so the job's total is silently 4x the truth. Nothing in the output says so; the numbers just look wrong in a way nobody notices.

What this does

codecarbon/core/slurm.py reads SLURM_NTASKS_PER_NODE at tracker init and, in machine mode with more than one rank per node, logs a warning naming the two ways out: elect one tracker per node (SLURM_LOCALID == 0) or switch to tracking_mode="process".

That is the whole change: ~20 lines, os.environ and a regex, no dependency, no config key, no new output field. 4(x2) (heterogeneous allocations) parses as 4.

Not in this PR

Automatic rank election. Suppressing the extra trackers is the real fix, but it is the same problem as LOCAL_RANK == 0 in distributed training and deserves one generic solution with a caller asking for it. Warning first.

Tests

tests/test_slurm.py: warns for 4 and for 4(x2); silent for 1, for no SLURM variable at all, and for tracking_mode="process".

uv run pytest tests/test_slurm.py tests/test_emissions_tracker.py -q
33 passed

Based on #1370, same base as #1366.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.45%. Comparing base (3ec31a0) to head (dae06ff).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1390      +/-   ##
==========================================
+ Coverage   91.43%   91.45%   +0.02%     
==========================================
  Files          49       50       +1     
  Lines        5057     5069      +12     
==========================================
+ Hits         4624     4636      +12     
  Misses        433      433              

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