[Bugfix][TPU] Fix TPU validate_request

[mgoin]

Fix due to #16291, we needed to actually import SamplingParams rather than just using it for TYPE_CHECKING

Without this change you hit this error when making a request on TPU in V1

  File "/home/mgoin/code/vllm/vllm/v1/engine/processor.py", line 186, in process_inputs
    current_platform.validate_request(
  File "/home/mgoin/code/vllm/vllm/platforms/tpu.py", line 157, in validate_request
    if isinstance(params,
       ^^^^^^^^^^^^^^^^^^
TypeError: isinstance() arg 2 must be a type, a tuple of types, or a union

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[njhill]

Thanks @mgoin

cc @joerunde