Covers the MPS Metal Graph compiler crash that motivated the fix:
adamw_step_fused crashed when p was bf16 (the standard nanochat
config for wte/value_embeds) but the optimizer's shared scalar
hyperparameters were fp32. Two tests:
- test_adamw_step_fused_bf16_param_with_fp32_scalars: smoke test
on bf16 path, verifies no crash and a finite weight update.
- test_adamw_step_fused_fp32_param_unchanged: confirms fp32 path
still produces a sensible update (the dtype-cast patch is a
no-op when source dtype matches target).
Both tests run on CPU (default) or MPS (when available). Muon's
mixed-dtype path is gated on the COMPUTE_DTYPE module constant
(set from NANOCHAT_DTYPE env var at import time), which is
awkward to exercise in a unit test without subprocess; the muon
fix is covered by manual end-to-end testing on M2 + bf16 instead.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
