Usage with pyarrow parquet

Hello, I'm very interested by the library usage however I struggle to apply it to a parquet file other than the dremel example.

````
from struct2tensor import expression_impl
import struct2tensor as s2t
import pyarrow as pa
import pyarrow.parquet as pq

tbl = pa.table([pa.array([0, 1])], names='a')
pq.ParquetWriter('/tmp/test', tbl.schema).write_table(tbl)
filenames = ["/tmp/test"]
batch_size = 2

exp = s2t.expression_impl.parquet.create_expression_from_parquet_file(filenames)
ps = exp.project(['a'])

val = s2t.expression_impl.parquet.calculate_parquet_values([ps], exp,
filenames, batch_size)
for h in val:
break
````

segfaults with the error:
2021-04-15 15:30:40.254237: E struct2tensor/kernels/parquet/parquet_reader.cc:198]
The repetition type of the root node was 0, but should be 2. There may be something wrong with your supplied parquet schema. We will treat it as a repeated field.

2021-04-15 15:31:46.428109: W tensorflow/core/framework/dataset.cc:477]
Input of ParquetDatasetOp::Dataset will not be optimized because the dataset does not implement the AsGraphDefInternal() method needed to apply optimizations.

I also tried saving again the dremel file loaded with Pyarrow and dumping it right away and I can reproduce the error.

How do you advise to save your parquet ?

Thanks for your help !

从 issue 中的 PyArrow reproducer 开始，检查 struct2tensor/kernels/parquet/parquet_reader.cc 第 198 行附近的代码，并将生成的 schema 与 dremel 示例进行比较。跟踪示例中展示的 parquet 表达式和值计算入口点；当使用 PyArrow 写入的 parquet 文件能够读取且不会出现报告中的 schema 错误或 segfault 时，即表示完成。

由索引模型根据 Issue 内容生成。