[TPU] Support Pathways in vLLM (#21417)

Signed-off-by: wenxindongwork <wenxindong@google.com>

vllm/envs.py

@@ -124,6 +124,7 @@ if TYPE_CHECKING:
     VLLM_V1_USE_OUTLINES_CACHE: bool = False
     VLLM_TPU_BUCKET_PADDING_GAP: int = 0
     VLLM_TPU_MOST_MODEL_LEN: Optional[int] = None
+    VLLM_TPU_USING_PATHWAYS: bool = False
     VLLM_USE_DEEP_GEMM: bool = False
     VLLM_USE_FLASHINFER_MOE_FP8: bool = False
     VLLM_USE_FLASHINFER_MOE_FP4: bool = False
@@ -900,6 +901,10 @@ environment_variables: dict[str, Callable[[], Any]] = {
     "VLLM_TPU_MOST_MODEL_LEN":
     lambda: maybe_convert_int(os.environ.get("VLLM_TPU_MOST_MODEL_LEN", None)),
 
+    # Whether using Pathways
+    "VLLM_TPU_USING_PATHWAYS":
+    lambda: bool("proxy" in os.getenv("JAX_PLATFORMS", "").lower()),
+
     # Allow use of DeepGemm kernels for fused moe ops.
     "VLLM_USE_DEEP_GEMM":
     lambda: bool(int(os.getenv("VLLM_USE_DEEP_GEMM", "0"))),