[hardware] unify usage of is_tpu to current_platform.is_tpu() (#7102)

vllm/platforms/__init__.py

@@ -1,22 +1,25 @@
-from typing import Optional
-
 import torch
 
-from vllm.utils import is_tpu
-
 from .interface import Platform, PlatformEnum, UnspecifiedPlatform
 
-current_platform: Optional[Platform]
+current_platform: Platform
 
-if torch.version.cuda is not None:
+try:
+    import libtpu
+except ImportError:
+    libtpu = None
+
+if libtpu is not None:
+    # people might install pytorch built with cuda but run on tpu
+    # so we need to check tpu first
+    from .tpu import TpuPlatform
+    current_platform = TpuPlatform()
+elif 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()
-elif is_tpu():
-    from .tpu import TpuPlatform
-    current_platform = TpuPlatform()
 else:
     current_platform = UnspecifiedPlatform()