Update Gemma on Dataflow example to pass max_length as arg

[jrmccluskey]

Description

Passes the max_length parameter to Gemma's generate() call through inference_args rather than having a hard-coded option.

Checklist

• [ ] I have followed Sample Guidelines from AUTHORING_GUIDE.MD
• [ ] README is updated to include all relevant information
• [ ] Tests pass: nox -s py-3.9 (see Test Environment Setup)
• [ ] Lint pass: nox -s lint (see Test Environment Setup)
• [ ] These samples need a new API enabled in testing projects to pass (let us know which ones)
• [ ] These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• [ ] This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• [ ] This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• [ ] Please merge this PR for me once it is approved

[jrmccluskey]

Got a clean run using this version of the code on a different GCP project since the test runs here have been hitting compute quotas