[Bugfix][Misc] Use TritonPlaceholderModule to defensively import triton (#15099)

Signed-off-by: Mengqing Cao <cmq0113@163.com>

vllm/utils.py

@@ -63,6 +63,9 @@ from torch.library import Library
 from typing_extensions import Never, ParamSpec, TypeIs, assert_never
 
 import vllm.envs as envs
+# NOTE: import triton_utils to make TritonPlaceholderModule work
+#       if triton is unavailable
+import vllm.triton_utils  # noqa: F401
 from vllm.logger import enable_trace_function_call, init_logger
 
 if TYPE_CHECKING: