Don't open an unnecessary init_scope in _cache_key

Also make the same improvement to executing_eagerly_outside_functions

Before:
entry {
  name: "MicroBenchmarks.benchmark_defun_matmul_2_by_2_CPU"
  iters: 30000
  wall_time: 54.0651639303
  extras {
    key: "examples_per_sec"
    value {
      double_value: 18496.1984262
    }
  }
}

After:
entry {
  name: "MicroBenchmarks.benchmark_defun_matmul_2_by_2_CPU"
  iters: 30000
  wall_time: 47.4757671356
  extras {
    key: "examples_per_sec"
    value {
      double_value: 21063.3773888
    }
  }
}
PiperOrigin-RevId: 224051558