Generate start and end indices per outfile.

def generate_shard_args(outfiles, num_examples):
  """Generate start and end indices per outfile."""
  num_shards = len(outfiles)
  num_examples_per_shard = num_examples // num_shards
  start_idxs = [i * num_examples_per_shard for i in range(num_shards)]
  end_idxs = list(start_idxs)
  end_idxs.pop(0)
  end_idxs.append(num_examples)
  return zip(start_idxs, end_idxs, outfiles)