Merge remote-tracking branch 'origin/main' into prep/pr-1112

# Conflicts:
#	CHANGELOG.md

backend/backends/base.py

@@ -6,6 +6,7 @@ voice prompt combination, and model loading progress tracking.
 """
 
 import logging
+import os
 import platform
 from contextlib import contextmanager
 from pathlib import Path
@@ -22,6 +23,24 @@ from ..utils.tasks import get_task_manager
 logger = logging.getLogger(__name__)
 
 
+def has_in_progress_download(blobs_dir: Path) -> bool:
+    """
+    Whether a HuggingFace repo's ``blobs`` dir holds a genuinely in-progress download.
+
+    An ``.incomplete`` blob means a download is still in progress -- unless a
+    completed blob with the same hash already sits next to it, which happens
+    when a retried/concurrent download leaves a stale ``.incomplete`` behind
+    after the real transfer already finished. Only orphaned ``.incomplete``
+    files (no matching completed blob) count as "in progress".
+    """
+    if not blobs_dir.exists():
+        return False
+    return any(
+        not incomplete.with_name(incomplete.name.removesuffix(".incomplete")).exists()
+        for incomplete in blobs_dir.glob("*.incomplete")
+    )
+
+
 def is_model_cached(
     hf_repo: str,
     *,
@@ -48,10 +67,8 @@ def is_model_cached(
         if not repo_cache.exists():
             return False
 
-        # Incomplete blobs mean a download is still in progress
-        blobs_dir = repo_cache / "blobs"
-        if blobs_dir.exists() and any(blobs_dir.glob("*.incomplete")):
-            logger.debug(f"Found .incomplete files for {hf_repo}")
+        if has_in_progress_download(repo_cache / "blobs"):
+            logger.debug(f"Found in-progress .incomplete file for {hf_repo}")
             return False
 
         snapshots_dir = repo_cache / "snapshots"
@@ -78,6 +95,13 @@ def is_model_cached(
         return False
 
 
+# Documented escape hatch (docs/content/docs/overview/gpu-acceleration.mdx):
+# users whose GPU has no compiled kernels in the bundled PyTorch set this to run
+# on CPU instead of crashing at generation time.
+FORCE_CPU_ENV_VAR = "VOICEBOX_FORCE_CPU"
+FORCE_CPU_ENABLED_VALUE = "1"
+
+
 def get_torch_device(
     *,
     allow_xpu: bool = False,
@@ -93,7 +117,17 @@ def get_torch_device(
         allow_directml: Check for DirectML (Windows) support.
         allow_mps: Allow MPS (Apple Silicon). If False, MPS falls back to CPU.
         force_cpu_on_mac: Force CPU on macOS regardless of GPU availability.
+
+    The VOICEBOX_FORCE_CPU override wins over every other candidate, and is
+    resolved before torch is imported so it still works when the installed
+    build is the reason CPU is wanted.
     """
+    # Stripped: on Windows, where this override matters most, it is usually set
+    # through the GUI environment editor.
+    if os.environ.get(FORCE_CPU_ENV_VAR, "").strip() == FORCE_CPU_ENABLED_VALUE:
+        logger.info("%s=%s set, forcing CPU device", FORCE_CPU_ENV_VAR, FORCE_CPU_ENABLED_VALUE)
+        return "cpu"
+
     if force_cpu_on_mac and platform.system() == "Darwin":
         return "cpu"