Add HIP_VISIBLE_DEVICES support, unit-aware VRAM parsing, Windows GPU validation

- hardware.py: check HIP_VISIBLE_DEVICES and ROCR_VISIBLE_DEVICES on ROCm
  before falling back to CUDA_VISIBLE_DEVICES, so multi-GPU AMD setups with
  HIP-specific env vars report the correct visible device set
- amd.py: add _parse_memory_mb() that reads "unit" from dict-shaped amd-smi
  JSON (e.g. {"value": 192, "unit": "GiB"}) and converts to MB correctly;
  fixes MI300X VRAM misreported as 0.19 GB instead of 192 GB
- install_python_stack.py: Windows AMD warning now validates actual GPU
  presence via hipinfo/amd-smi output markers before printing
- install_llama_prebuilt.py: restore amdhip64.dll fallback for Windows HIP
  detection after tool-based checks, so Windows HIP installs without CLI
  tools on PATH are still detected
- hardware.py: fix IS_ROCM comment to accurately describe its role

studio/install_python_stack.py

@@ -726,7 +726,29 @@ def install_python_stack() -> int:
     # Windows + AMD GPU: PyTorch does not publish ROCm wheels for Windows.
     # Detect and warn so users know manual steps are needed for GPU training.
     if IS_WINDOWS and not NO_TORCH and not _has_usable_nvidia_gpu():
-        if shutil.which("hipinfo") or shutil.which("amd-smi"):
+        # Validate actual AMD GPU presence (not just tool existence)
+        _win_amd_gpu = False
+        for _wcmd, _wmarker in (
+            (["hipinfo"], "gcnarchname"),
+            (["amd-smi", "list"], "gpu"),
+        ):
+            _wexe = shutil.which(_wcmd[0])
+            if not _wexe:
+                continue
+            try:
+                _wr = subprocess.run(
+                    [_wexe, *_wcmd[1:]],
+                    stdout=subprocess.PIPE,
+                    stderr=subprocess.DEVNULL,
+                    text=True,
+                    timeout=10,
+                )
+            except Exception:
+                continue
+            if _wr.returncode == 0 and _wmarker in _wr.stdout.lower():
+                _win_amd_gpu = True
+                break
+        if _win_amd_gpu:
             _safe_print(
                 _dim("  Note:"),
                 "AMD GPU detected on Windows. ROCm-enabled PyTorch must be",