Stop defaulting number of eager threads to 1.

Defaulting to 1 is good for microbenchmarks, but bad for larger functions.

Before:
entry {
  name: "MicroBenchmarks.benchmark_keras_model_subclassed_fit"
  iters: 1
  wall_time: 907522.916794
  extras {
    key: "examples_per_sec"
    value {
      double_value: 1.10190054873
    }
  }
}

After:
entry {
  name: "MicroBenchmarks.benchmark_keras_model_subclassed_fit"
  iters: 1
  wall_time: 805292.129517
  extras {
    key: "examples_per_sec"
    value {
      double_value: 1.24178538861
    }
  }
}
PiperOrigin-RevId: 221469005