feat: make out dim a lazy assignment

[stephantul]

The out dimensionality of a trainable model was set, but was then always reset on fitting. This is a breaking change, which will be noted in patch notes on a new release. The breaking change arguably only applies to users who manually set out_dim, which has no effect.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
model2vec/train/base.py	100.00% <100.00%> (ø)
model2vec/train/classifier.py	100.00% <100.00%> (ø)
model2vec/train/similarity.py	100.00% <ø> (ø)

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[greptile-apps]

Confidence Score: 4/5

[Medium impact] Changes when model heads are constructed during initialization.

The PR appears safe to merge, with two non-blocking corrections to its initialization guidance.

_{Reviews (1) · Last reviewed commit: "make out dim a lazy assignment" · Reviewed by Greptile}