PR #25929: Fix warning in threadpool_dataset_op.cc

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Imported from GitHub PR #25929

num_threads should be greater than 1 and its initialised with 0

Copybara import of the project:

  - 0f4cb7537755c3ccba5a2d68314cdf7b9885307c Fix warning in threadpool_dataset_op.cc by Albin Joy <albin.joy@huawei.com>
  - 730e2d6066a5f54de76088fd7c822de0d991bc59 Merge 0f4cb7537755c3ccba5a2d68314cdf7b9885307c into 57e8d... by Albin Joy <albin.joy@huawei.com>

COPYBARA_INTEGRATE_REVIEW=https://github.com/tensorflow/tensorflow/pull/25929 from joyalbin:fix_threadpool_dataset_op 0f4cb7537755c3ccba5a2d68314cdf7b9885307c
PiperOrigin-RevId: 235761317