[hardware][misc] introduce platform abstraction (#6080)

vllm/platforms/__init__.py

@@ -0,0 +1,18 @@
+from typing import Optional
+
+import torch
+
+from .interface import Platform, PlatformEnum
+
+current_platform: Optional[Platform]
+
+if torch.version.cuda is not None:
+    from .cuda import CudaPlatform
+    current_platform = CudaPlatform()
+elif torch.version.hip is not None:
+    from .rocm import RocmPlatform
+    current_platform = RocmPlatform()
+else:
+    current_platform = None
+
+__all__ = ['Platform', 'PlatformEnum', 'current_platform']