Aggregate Functions - CombineGlobally and CombinePerKey not working for side inputs (either for Singleton, AsIter, AsDict

Hi,

Python examples for Aggregate transformer - CombineGlobally and CombiePerKey doesn't work for side inputs (singleton, iter and dict). Even when used with custom class inheriting beam.CombineFn it fails.

Third combine transformer - i.e., CombineValues is working fine without any issue.

Steps to recreate problem:

Python version 3.7.3

Apache beam = 2.24.0 (installed with gcp data flow components)

Imported from Jira [BEAM-11185](https://issues.apache.org/jira/browse/BEAM-11185). Original Jira may contain additional context.
Reported by: aravindh.venkatesh.