Something wrong in text_parser.h

[Lisz1024]

  const int nthread = omp_get_max_threads();
  data->resize(nthread);
  ...
  #pragma omp parallel num_threads(nthread_)
  {
    // threadid
    int tid = omp_get_thread_num();
    size_t nstep = (chunk.size + nthread - 1) / nthread;
   ...
  }

Why the num_threads is nthread_ , buy use nthread to calculate nstep?
In my machine, nthread_ = 2 and nthread = 20 , so I only can read 1/10 data, 18 data blocks are empty ...
Why not only use one of them? @tqchen

[tqchen]

good point, can you open a PR to fix this problem?