Fix warning KeyError: 'gpu_index'

[benoit-cty]

Description

Fix warning in tests/test_emissions_tracker.py::TestCarbonTracker::test_carbon_tracker_TWO_GPU_PRIVATE_INFRA_CANADA:

=============================== warnings summary ===============================
tests/test_emissions_tracker.py::TestCarbonTracker::test_carbon_tracker_TWO_GPU_PRIVATE_INFRA_CANADA
  /home/runner/work/codecarbon/codecarbon/.venv/lib/python3.12/site-packages/_pytest/threadexception.py:58: PytestUnhandledThreadExceptionWarning: Exception in thread Thread-17
  
  Traceback (most recent call last):
    File "/home/runner/.local/share/uv/python/cpython-3.12.13-linux-x86_64-gnu/lib/python3.12/threading.py", line 1075, in _bootstrap_inner
      self.run()
    File "/home/runner/.local/share/uv/python/cpython-3.12.13-linux-x86_64-gnu/lib/python3.12/threading.py", line 1433, in run
      self.function(*self.args, **self.kwargs)
    File "/home/runner/work/codecarbon/codecarbon/.venv/lib/python3.12/site-packages/codecarbon/external/scheduler.py", line 42, in _run
      self.function(*self.args, **self.kwargs)
    File "/home/runner/work/codecarbon/codecarbon/.venv/lib/python3.12/site-packages/codecarbon/emissions_tracker.py", line 983, in _monitor_power
      gpu_detail["gpu_index"] in gpu_ids_to_monitor
      ~~~~~~~~~~^^^^^^^^^^^^^
  KeyError: 'gpu_index'

Motivation and Context

It's only a warning but it looks like an error.

The thread warning was caused by the background GPU utilization sampler assuming every GPU detail dict contains gpu_index. I fixed that in emissions_tracker.py:982 by falling back to the position in the returned GPU details list when the key is missing, which keeps subset filtering working and avoids the KeyError.

How Has This Been Tested?

❯ uv run python -m pytest tests/test_emissions_tracker.py
=========================================================== test session starts ===========================================================
platform linux -- Python 3.13.7, pytest-9.0.2, pluggy-1.6.0
rootdir: /home/benoit/CODECARBON/codecarbon
configfile: pytest.ini (WARNING: ignoring pytest config in pyproject.toml!)
plugins: logfire-4.29.0, cov-7.0.0, requests-mock-1.12.1
collected 20 items                                                                                                                        

tests/test_emissions_tracker.py ....................                                                                                [100%]

=========================================================== 20 passed in 25.02s =================================

Types of changes

What types of changes does your code introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

Go over all the following points, and put an x in all the boxes that apply.

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING.md document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.74%. Comparing base (7ea4965) to head (9aa4910).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1122      +/-   ##
==========================================
+ Coverage   80.71%   80.74%   +0.03%     
==========================================
  Files          41       41              
  Lines        3920     3921       +1     
==========================================
+ Hits         3164     3166       +2     
+ Misses        756      755       -1     

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[inimaz]

Thanks @benoit-cty !