fix: only allows named args in pretrain

[stephantul]

#199 shows that it is confusing that out_dim comes before the model_name in from_pretrained. So fix this, I switched to only allowing named arguments in from_pretrained. I explicitly did not change it in the other parts of the code base because confusion is less apparent there.

[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)
tests/test_trainable.py	100.00% <100.00%> (ø)