fix: integrate GPU power when energy counter is absent

[davidberenstein1957]

What

GPUDevice.delta now branches on the backend returning None for the cumulative energy counter. When it does, it computes Power.from_watts(self._get_power_usage()) and Energy.from_power_and_time(...) instead of comparing two identical counter readings, and logs a single warning per device so users know which method produced their numbers.

The per-sample logger.warning in NvidiaGPUDevice._get_total_energy_consumption is demoted to debug, since the once-per-device warning now carries the signal.

Why

On GPUs where NVML does not implement nvmlDeviceGetTotalEnergyConsumption (pre-Volta cards, many virtualised GPUs) the previous code returned last_energy unchanged, yielding energy_delta = 0 and power = 0 for the entire run — a GPU-bound job silently reported only CPU + RAM. The fallback lives in the shared GPUDevice, so the AMD backend (which returns None when no energy accumulator is present) is fixed by the same change.

Power integration is less accurate than the counter, but it is the same approximation the CPU path already uses and is dramatically better than zero.

How verified

tests/test_gpu_nvidia.py::TestGpu::test_gpu_without_energy_counter_falls_back_to_power patches the NVML energy query to raise and asserts 26 W / 29 W and 0.026 / 0.029 kWh over an hour. It fails on master (assert 0.0 == 26) and passes here. tests/test_gpu.py, tests/test_gpu_nvidia.py and tests/test_gpu_amd.py all pass (51 tests).

Closes #1316

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.48%. Comparing base (3ec31a0) to head (1889fda).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1319      +/-   ##
==========================================
+ Coverage   91.43%   91.48%   +0.04%     
==========================================
  Files          49       49              
  Lines        5057     5071      +14     
==========================================
+ Hits         4624     4639      +15     
+ Misses        433      432       -1     

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