Make grpc-requiring class definition optional.

[robertwb]

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Post-Commit Tests Status (on master branch)

Lang	SDK	Apex	Dataflow	Flink	Gearpump	Samza	Spark
Go		---	---		---	---
Java
Python		---			---	---
XLang	---	---	---		---	---

Pre-Commit Tests Status (on master branch)

---	Java	Python	Go	Website
Non-portable
Portable	---		---	---

See .test-infra/jenkins/README for trigger phrase, status and link of all Jenkins jobs.

[kennknowles]

Should there be a tracking Jira to clean this stuff up? Is there a linter that could catch such things? Etc?

[kennknowles]

Run Python PreCommit

[robertwb]

This is a google-only constraint; there is an internal bug tracking this. (I cc'd you.) I also created https://issues.apache.org/jira/browse/BEAM-9955 for the external cleanup.