Add shape validation for tag input of StatsDataset (#18462)

* Add shape validation for tag input of StatsDataset

The tag field of the StatsDataset needs to be a scalar.
However, there was no check in the shape function. This
fix adds the check of the tag shape.

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

* Add test case for tag shape check with BytesProducedStatsDataset

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

* Add tag shape check for LatencyStatsDataset

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

* Add test case for tag shape check with LatencyStatsDataset

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

* Fix pylint issue

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