refactor: remove redundant .to(device) in StageNet and convert assert to ValueError

[haoyu-haoyu]

Summary

• Remove 9 redundant .to(device) calls from StageNet.step() loop (called T times per forward pass)
• Convert input-validation assert statements to ValueError in user-facing modules

Changes

Performance: stagenet_mha.py

The step() method is called once per timestep in the forward loop. It contained 9 .to(device=device) calls on intermediate tensors that already inherit device from their inputs. Removing them eliminates ~9T unnecessary operations per forward pass.

Code quality: assert → ValueError

Converted input-validation assertions to proper ValueError exceptions in:

File	Assertions converted
datasets/splitter.py	19 (ratio validation, data type checks)
calib/calibration/hb.py	6 (parameter bounds)
tasks/readmission_prediction.py	5 (window/age validation)
tasks/sleep_staging.py	4 (epoch/channel validation)
tasks/benchmark_ehrshot.py	1
tasks/covid19_cxr_classification.py	1
tasks/in_hospital_mortality_mimic4.py	1
tasks/medical_transcriptions_classification.py	1

Internal invariants (red-black tree, model framework checks) are intentionally left as assert.

Test plan

• CI passes
• python -m pytest tests/ passes
• No semantic changes — only error type changes from AssertionError to ValueError