Make number of training samples per year configurable for FCN AFNO example

[ktangsali]

Modulus Pull Request

Small change to enable QA.

Checklist

• [x] I am familiar with the Contributing Guidelines.
• [x] New or existing tests cover these changes.
• [x] The documentation is up to date with these changes.
• [x] The CHANGELOG.md is up to date with these changes.

Dependencies

[ktangsali]

/blossom-ci