[TPU] Fix import error in tpu launch (#28758)

Signed-off-by: Qiliang Cui <derrhein@gmail.com>

vllm/platforms/tpu.py

@@ -9,20 +9,25 @@ from tpu_info import device
 
 from vllm.inputs import ProcessorInputs, PromptType
 from vllm.logger import init_logger
-from vllm.sampling_params import SamplingParams, SamplingType
 
 from .interface import Platform, PlatformEnum
 
 if TYPE_CHECKING:
+    from typing import TypeAlias
+
     from vllm.attention.backends.registry import AttentionBackendEnum
     from vllm.config import VllmConfig
     from vllm.config.cache import BlockSize
     from vllm.pooling_params import PoolingParams
+    from vllm.sampling_params import SamplingParams
+
+    ParamsType: TypeAlias = SamplingParams | PoolingParams
 else:
     BlockSize = None
     VllmConfig = None
     PoolingParams = None
     AttentionBackendEnum = None
+    ParamsType = None
 
 logger = init_logger(__name__)
 
@@ -203,10 +208,12 @@ class TpuPlatform(Platform):
     def validate_request(
         cls,
         prompt: PromptType,
-        params: SamplingParams | PoolingParams,
+        params: ParamsType,
         processed_inputs: ProcessorInputs,
     ) -> None:
         """Raises if this request is unsupported on this platform"""
+        from vllm.sampling_params import SamplingParams, SamplingType
+
         if (
             isinstance(params, SamplingParams)
             and params.sampling_type == SamplingType.RANDOM_SEED