Move PeriodicStream into periodicsequence. (#35412)

* Move PeriodicStream into periodicsequence.

* Consolidate PeriodicStream into PeriodicImpulse.

* Fix fn_runner_test that used PeriodicStream.

* Reformat

* Fix some edge cases and resolve floating point precision problem.

Add tests.

* Fix lint and move tests for periodic impulse to its own test case.

* Add more docstrings. Revise stop_timestamp part as it is an exclusive upper bound.

* Fix pydoc error.

Author: shunping

sdks/python/apache_beam/ml/ts/util.py

@@ -53,7 +53,6 @@
 from apache_beam.metrics import monitoring_infos
 from apache_beam.metrics.execution import MetricKey
 from apache_beam.metrics.metricbase import MetricName
-from apache_beam.ml.ts.util import PeriodicStream
 from apache_beam.options.pipeline_options import DebugOptions
 from apache_beam.options.pipeline_options import DirectOptions
 from apache_beam.options.pipeline_options import PipelineOptions
@@ -1264,7 +1263,7 @@ def test_sliding_windows(self):
     with self.create_pipeline() as p:
       ret = (
           p
-          | PeriodicStream(data, interval=1)
+          | PeriodicImpulse(data=data, fire_interval=1)
           | beam.WithKeys(0)
           | beam.WindowInto(beam.transforms.window.SlidingWindows(6, 3))
           | beam.GroupByKey())