Increase code test coverage to 80%

[emapuljak]

Description

Combined test coverage of src/ is currently 43% (measured across both backends with --cov-append), well below the 80% target. The fail_under gate is set low provisionally so CI can go green; this issue tracks backfilling tests and raising the gate to 80%.

Note: 43% is a floor — it was measured with ~121 tests failing (CUDA/backend-gating issues). Re-measure once the suite is green before setting intermediate targets.

Biggest gaps

Module	Coverage	Statements
core/torch/fit_compress.py	0%	474
core/torch/distillers.py	0%	315
core/torch/pruning_methods/fitcompress.py	0%	33
core/utils.py	0%	41
core/torch/train.py	6%	38
core/torch/optimizers.py	14%	45
core/torch/layers.py	48%	1244

fit_compress.py + distillers.py alone are ~790 untested statements (~8.5% of the codebase).

Tasks

• Re-measure combined coverage once the test suite passes cleanly.
• Add tests for the FitCompress path (fit_compress.py, pruning_methods/fitcompress.py).
• Add tests for distillers.py.
• Cover core/utils.py, core/torch/train.py, core/torch/optimizers.py.
• Raise fail_under in pyproject.toml incrementally (e.g. 60 → 80) as coverage lands.

Acceptance Criteria

• Combined coverage across both backends is ≥ 80%.
• [tool.coverage.report] fail_under = 80 and the CI coverage job passes.
• No module listed above remains at 0%.