feat(strixhalo): patch torch.cuda.mem_get_info for unified memory APU

On Strix Halo, PyTorch reports GTT pool (128 GiB) as device memory
instead of real VRAM (96 GiB from BIOS). vLLM uses mem_get_info() to
pre-allocate and refuses to start when free GTT (29 GiB) < requested.

The strixhalo_vram_fix.pth hook auto-patches mem_get_info on Python
startup to read real VRAM total/used from /sys/class/drm sysfs.
Only activates when PyTorch total differs >10% from sysfs VRAM.

amdsmi-shim/strixhalo_vram_fix.py

@@ -0,0 +1,107 @@
+"""
+Monkey-patch torch.cuda.mem_get_info for AMD APUs with unified memory.
+
+On Strix Halo (and other AMD APUs), torch.cuda.mem_get_info() returns
+GTT (system RAM accessible to GPU) numbers instead of actual VRAM:
+  - Total:  128 GiB  (GTT pool, NOT real VRAM)
+  - Free:    29 GiB  (GTT free, NOT VRAM free)
+
+Meanwhile sysfs reports the real VRAM:
+  - Total:  96 GiB   (actual dedicated VRAM from BIOS)
+  - Free:   95.8 GiB (actual VRAM free)
+
+vLLM uses torch.cuda.mem_get_info() to calculate how much memory to
+pre-allocate.  With wrong numbers it either OOMs or refuses to start.
+
+This module patches mem_get_info to return sysfs VRAM values, which
+are the physically meaningful numbers for allocation decisions.
+
+Installed as a .pth hook so it runs before any user code.
+"""
+
+import os
+import glob
+import logging
+
+logger = logging.getLogger("strixhalo_vram_fix")
+
+
+def _read_sysfs_int(path: str) -> int | None:
+    try:
+        with open(path) as f:
+            return int(f.read().strip())
+    except (OSError, ValueError):
+        return None
+
+
+def _get_real_vram() -> tuple[int, int] | None:
+    """Read real VRAM total/used from sysfs for the first AMD GPU."""
+    for card_dir in sorted(glob.glob("/sys/class/drm/card[0-9]*/device")):
+        vendor_path = os.path.join(card_dir, "vendor")
+        if not os.path.exists(vendor_path):
+            continue
+        try:
+            with open(vendor_path) as f:
+                vendor = f.read().strip()
+        except OSError:
+            continue
+        if vendor != "0x1002":  # AMD
+            continue
+
+        total = _read_sysfs_int(os.path.join(card_dir, "mem_info_vram_total"))
+        used = _read_sysfs_int(os.path.join(card_dir, "mem_info_vram_used"))
+        if total is not None and used is not None:
+            return (total, used)
+    return None
+
+
+def _apply_patch() -> None:
+    """Patch torch.cuda.mem_get_info if we detect unified memory mis-reporting."""
+    try:
+        import torch
+        if not hasattr(torch, "cuda") or not torch.cuda.is_available():
+            return
+    except ImportError:
+        return
+
+    vram_info = _get_real_vram()
+    if vram_info is None:
+        return
+
+    vram_total, vram_used = vram_info
+    vram_free = vram_total - vram_used
+
+    # Only patch if PyTorch total differs significantly from sysfs VRAM
+    # (i.e. PyTorch is reporting GTT/unified memory, not real VRAM)
+    try:
+        pt_free, pt_total = torch.cuda.mem_get_info(0)
+    except Exception:
+        return
+
+    # If they're within 10% of each other, no patch needed
+    if abs(pt_total - vram_total) / max(pt_total, 1) < 0.10:
+        return
+
+    original_fn = torch.cuda.mem_get_info
+
+    def _patched_mem_get_info(device=None):
+        """Return real VRAM from sysfs instead of GTT numbers."""
+        real = _get_real_vram()
+        if real is None:
+            return original_fn(device)
+        total, used = real
+        # Account for PyTorch's own allocations on top of sysfs baseline
+        pt_allocated = torch.cuda.memory_allocated(device or 0)
+        free = total - used - pt_allocated
+        return (max(free, 0), total)
+
+    torch.cuda.mem_get_info = _patched_mem_get_info
+    logger.info(
+        "strixhalo_vram_fix: patched torch.cuda.mem_get_info "
+        "(PyTorch reported %d GiB total, sysfs VRAM is %d GiB)",
+        pt_total // (1024**3),
+        vram_total // (1024**3),
+    )
+
+
+_apply_patch()