Enhance estimator predict with RayXshards (#4211)
Enhance estimator predict with RayXshards (#4211)

* remove is_empty in write_to_ray return result_rdd

* add result_rdd

* change rayxshards constructor signature (rdd)

* use self.rdd in to_spark_xshards
intel-analytics/analytics-zoo,intel-analytics/analytics-zoo,intel-analytics/analytics-zoo