fix(device): correct attribute name from total_mem to total_memory

torch.cuda.get_device_properties() exposes total_memory, not total_mem.
This caused an AttributeError when calling detect_gpu_info() or auto_configure().

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

llm_lab/utils/device.py

@@ -24,7 +24,7 @@ def detect_gpu_info() -> dict:
         return {}
     return {
         "name": torch.cuda.get_device_name(),
-        "memory_gb": round(torch.cuda.get_device_properties(0).total_mem / 1e9, 1),
+        "memory_gb": round(torch.cuda.get_device_properties(0).total_memory / 1e9, 1),
     }
 
 
@@ -45,7 +45,7 @@ def auto_configure(config: "TrainConfig") -> "TrainConfig":
         return config
 
     gpu_name = torch.cuda.get_device_name().lower()
-    gpu_mem = torch.cuda.get_device_properties(0).total_mem / 1e9
+    gpu_mem = torch.cuda.get_device_properties(0).total_memory / 1e9
 
     print(f"\n🔍 GPU detected: {torch.cuda.get_device_name()} ({gpu_mem:.1f} GB)")