[Bug]: Google Cloud Storage Sink not working with unbounded pcollection using Sliding window and GrouByKey

### What happened?

Take an unbounded pcollection
Attach event timestamp and sliding window
Per element, in order to find count of elements with same id over the last X hr/min/sec.. this will require CombinePerKey and CoGroupByKey at different stages
Write the output of CoGroupByKey to GCS

If I write the output to BQ, it works fine but with GCS, it gives the following error

`ValueError: GroupByKey cannot be applied to an unbounded PCollection with global windowing and a default trigger`

I am not even using a global window but instead a sliding window so this definitely seems like a bug.

### Issue Priority

Priority: 3 (minor)

### Issue Components

- [X] Component: Python SDK
- [ ] Component: Java SDK
- [ ] Component: Go SDK
- [ ] Component: Typescript SDK
- [ ] Component: IO connector
- [ ] Component: Beam examples
- [ ] Component: Beam playground
- [ ] Component: Beam katas
- [ ] Component: Website
- [ ] Component: Spark Runner
- [ ] Component: Flink Runner
- [ ] Component: Samza Runner
- [ ] Component: Twister2 Runner
- [ ] Component: Hazelcast Jet Runner
- [X] Component: Google Cloud Dataflow Runner