Dummy models for CI

[javiermtorres]

For CI purposes, we would need dummy models that return a predictable (e.g. constant) value, with as little processing as possible. These will honor the general interface (token classification, sequence classification, etc) but just return a dummy value. This allows testing integration and non-ML related changes efficiently.

Closes #98

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
encoderfile/src/build_cli/tokenizer.rs	92.73% <100.00%> (+1.28%)	⬆️
...i/transforms/validation/sequence_classification.rs	89.61% <100.00%> (+0.42%)	⬆️
..._cli/transforms/validation/token_classification.rs	89.18% <100.00%> (ø)

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