[Bugfix] Fix triton import with local TritonPlaceholder (#17446)

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

vllm/triton_utils/__init__.py

@@ -1,5 +1,13 @@
 # SPDX-License-Identifier: Apache-2.0
 
-from vllm.triton_utils.importing import HAS_TRITON
+from vllm.triton_utils.importing import (HAS_TRITON, TritonLanguagePlaceholder,
+                                         TritonPlaceholder)
 
-__all__ = ["HAS_TRITON"]
+if HAS_TRITON:
+    import triton
+    import triton.language as tl
+else:
+    triton = TritonPlaceholder()
+    tl = TritonLanguagePlaceholder()
+
+__all__ = ["HAS_TRITON", "triton", "tl"]