fix: makes fit_transform behavior consistent with fit and transform

[ipanepen]

The behavior of fit_transform was inconsistent with behavior of fit and transform run sequentially. It should be possible to use fit_transform for the normal functionality of RobustLabelEncoder (filling unseen values). I modified already existing tests, and created a new one to ensure the behavior is as expected.

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[wiltonwu]

AWS CodeBuild CI Report

• CodeBuild project: sagemaker-sklearn-extension-pr
• Commit ID: 9d683b2f58aa90861596357e9418af1b50a955ba
• Result: SUCCEEDED
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