Expose TFRecordWriter.Flush() in Python (#10686)

* Expose TFRecordWriter.Flush() in Python

This fix tries to address the issue raised in #10644 where
it was not possible to call `TFRecordWriter.Flush()` in Python.
The C++ interface already has the `Flush()` so this fix exposes
it to python level.

This fix is related to #10644.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Update the API goldens for APi compatibility

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>