fix(mps): align VOXCPM_MPS_DTYPE override set with get_dtype parser

Drop "half" from _VALID_DTYPE_OVERRIDES / _LOW_PRECISION_DTYPES.
get_dtype() has never accepted "half", so VOXCPM_MPS_DTYPE=half would
pass override validation and then crash downstream with
"Unsupported dtype: half". The remaining aliases (bfloat16/bf16,
float16/fp16, float32/fp32) already cover the intended dtype space.

Adds a standalone unit check under scripts/ to guard the invariant
that every accepted override parses through get_dtype().

Addresses review feedback on #263.

scripts/test_pick_runtime_dtype.py

@@ -0,0 +1,108 @@
+"""Unit checks for pick_runtime_dtype / get_dtype consistency.
+
+Loads src/voxcpm/model/utils.py directly to avoid the heavy voxcpm package
+init. Run with: `python scripts/test_pick_runtime_dtype.py`.
+"""
+import importlib.util
+import os
+import pathlib
+import sys
+
+REPO_ROOT = pathlib.Path(__file__).resolve().parent.parent
+UTILS = str(REPO_ROOT / "src" / "voxcpm" / "model" / "utils.py")
+spec = importlib.util.spec_from_file_location("voxcpm_utils", UTILS)
+utils = importlib.util.module_from_spec(spec)
+spec.loader.exec_module(utils)
+
+_LOW_PRECISION_DTYPES = utils._LOW_PRECISION_DTYPES
+_VALID_DTYPE_OVERRIDES = utils._VALID_DTYPE_OVERRIDES
+get_dtype = utils.get_dtype
+pick_runtime_dtype = utils.pick_runtime_dtype
+
+
+def expect(actual, expected, label):
+    ok = actual == expected
+    mark = "OK " if ok else "FAIL"
+    print(f"[{mark}] {label}: got={actual!r} expected={expected!r}")
+    return ok
+
+
+def expect_raises(fn, exc_type, label):
+    try:
+        fn()
+    except exc_type as e:
+        print(f"[OK ] {label}: raised {exc_type.__name__}: {e}")
+        return True
+    except Exception as e:
+        print(f"[FAIL] {label}: raised {type(e).__name__} not {exc_type.__name__}: {e}")
+        return False
+    print(f"[FAIL] {label}: no exception raised")
+    return False
+
+
+results = []
+
+print("=== override set sanity ===")
+results.append(expect("half" not in _VALID_DTYPE_OVERRIDES, True, "half removed from _VALID_DTYPE_OVERRIDES"))
+results.append(expect("half" not in _LOW_PRECISION_DTYPES, True, "half removed from _LOW_PRECISION_DTYPES"))
+
+print("\n=== every accepted override parses through get_dtype ===")
+for dt in sorted(_VALID_DTYPE_OVERRIDES):
+    try:
+        torch_dtype = get_dtype(dt)
+        print(f"[OK ] get_dtype({dt!r}) -> {torch_dtype}")
+        results.append(True)
+    except Exception as e:
+        print(f"[FAIL] get_dtype({dt!r}) raised: {e}")
+        results.append(False)
+
+print("\n=== pick_runtime_dtype: non-mps is a no-op ===")
+results.append(expect(pick_runtime_dtype("cuda", "bfloat16"), "bfloat16", "cuda/bf16 untouched"))
+results.append(expect(pick_runtime_dtype("cpu", "float16"), "float16", "cpu/fp16 untouched"))
+results.append(expect(pick_runtime_dtype("cuda", "float32"), "float32", "cuda/fp32 untouched"))
+
+print("\n=== pick_runtime_dtype: mps forces fp32 for low-precision ===")
+os.environ.pop("VOXCPM_MPS_DTYPE", None)
+results.append(expect(pick_runtime_dtype("mps", "bfloat16"), "float32", "mps/bf16 -> fp32"))
+results.append(expect(pick_runtime_dtype("mps", "bf16"), "float32", "mps/bf16-alias -> fp32"))
+results.append(expect(pick_runtime_dtype("mps", "float16"), "float32", "mps/fp16 -> fp32"))
+results.append(expect(pick_runtime_dtype("mps", "fp16"), "float32", "mps/fp16-alias -> fp32"))
+results.append(expect(pick_runtime_dtype("mps", "float32"), "float32", "mps/fp32 stays"))
+results.append(expect(pick_runtime_dtype("mps", "fp32"), "fp32", "mps/fp32-alias stays"))
+
+print("\n=== pick_runtime_dtype: VOXCPM_MPS_DTYPE override ===")
+os.environ["VOXCPM_MPS_DTYPE"] = "bfloat16"
+results.append(expect(pick_runtime_dtype("mps", "bfloat16"), "bfloat16", "override bf16 honored"))
+
+os.environ["VOXCPM_MPS_DTYPE"] = "FP16"
+results.append(expect(pick_runtime_dtype("mps", "bfloat16"), "fp16", "override is case-insensitive"))
+
+os.environ["VOXCPM_MPS_DTYPE"] = "  float32  "
+results.append(expect(pick_runtime_dtype("mps", "bfloat16"), "float32", "override is whitespace-trimmed"))
+
+print("\n=== pick_runtime_dtype: 'half' is no longer a valid override ===")
+os.environ["VOXCPM_MPS_DTYPE"] = "half"
+results.append(
+    expect_raises(
+        lambda: pick_runtime_dtype("mps", "bfloat16"),
+        ValueError,
+        "override=half now rejected (was the bug)",
+    )
+)
+
+os.environ["VOXCPM_MPS_DTYPE"] = "garbage"
+results.append(
+    expect_raises(
+        lambda: pick_runtime_dtype("mps", "bfloat16"),
+        ValueError,
+        "override=garbage still rejected",
+    )
+)
+
+os.environ.pop("VOXCPM_MPS_DTYPE", None)
+
+print("\n=== summary ===")
+passed = sum(results)
+total = len(results)
+print(f"{passed}/{total} passed")
+sys.exit(0 if passed == total else 1)

src/voxcpm/model/utils.py

@@ -3,10 +3,10 @@ from typing import List, Optional
 import torch
 from transformers import PreTrainedTokenizer
 
-_LOW_PRECISION_DTYPES = {"bfloat16", "bf16", "float16", "fp16", "half"}
+_LOW_PRECISION_DTYPES = {"bfloat16", "bf16", "float16", "fp16"}
 _VALID_DTYPE_OVERRIDES = {
     "bfloat16", "bf16",
-    "float16", "fp16", "half",
+    "float16", "fp16",
     "float32", "fp32",
 }