[Feature request]: Remove obsolete encoder_only arguments from model definitions

[laserkelvin]

Feature/behavior summary

With #67, model forward methods are expected to output Embeddings objects, which doesn't leave room for ambiguity on what the model outputs are since the output heads from tasks are meant to do the final projections.

Request attributes

• Would this be a refactor of existing code?
• Does this proposal require new package dependencies?
• Would this change break backwards compatibility?
• Does this proposal include a new model?
• Does this proposal include a new dataset?
• Does this proposal include a new task/workflow?

Related issues

#67 is where this was brought up by @melo-gonzo, based on the model output refactors.

Solution description

Remove encoder_only arguments from models that have them, and any associated projection layers in models that won't be used.

Additional notes

No response