examples/kws: replace module-level assert KWS_CLASSES validation with explicit raise

[LeoBuron]

Nit

The KWS examples validate the KWS_CLASSES env knob with a module-level assert:

NUM_CLASSES = int(os.environ.get("KWS_CLASSES", "6"))
assert NUM_CLASSES in (6, 35), NUM_CLASSES

assert is stripped under python -O / PYTHONOPTIMIZE, so the validation silently vanishes in optimized runs (an invalid KWS_CLASSES would then fail later with a confusing error).

Fix

Replace with an explicit guard that always runs:

if NUM_CLASSES not in (6, 35):
    raise ValueError(f"KWS_CLASSES must be 6 or 35, got {NUM_CLASSES}")

Affected files

• examples/kws_mfcc/{prepare_data,train_pytorch,compare}.py
• examples/kws_raw/{prepare_data,train_pytorch,compare}.py

(The C trainers already use an explicit if … fprintf … return default in readNumClasses(), so only the Python side needs this.)

Low priority — CI does not pass -O, so this is robustness/hygiene, not a live bug. Surfaced in the PR #256 final review.