fix: avoid eager estimator fallback lookup

[blackdragoon26]

What type of PR is this?

/kind bug

What this PR does / why we need it:

Selects fit() or the fallback method lazily in BackendBase.train() and BackendBase.update(). Fit-only, train-only, and update-only estimators no longer fail during eager fallback attribute lookup.

Which issue(s) this PR fixes:

Fixes #490

Validation:

Added lib/tests/test_backend_base.py with focused cases covering:

• train() with a fit-only estimator;
• train() falling back to train();
• update() with a fit-only estimator;
• update() falling back to update().

PYTHONPATH=lib python -m unittest discover -s lib/tests -v

Result: 4 tests passed. Python compilation and git diff --check also pass.

[kubeedge-bot]

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This pull-request has been approved by: blackdragoon26
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[blackdragoon26]

Kindly review this PR
@JimmyYang20 @TymonXie