Enable support for adding chat templates to base models

[RobotSail]

Today in order to train with an arbitrary model, its tokenizer must already have a chat template and an eos_token set.

But in if you want to train a base model with a chat template, you must use our hard-coded chat templates.

We want to implement the ability to give a base model an arbitrary chat template through the following methods:

• specifying an existing instruct model to use as the target
• providing some minimal metadata for the model to use (chat template + eos_token + pad token)

This will allow us to fully deprecate the hard-coded chat template system.

[github-actions]

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[github-actions]

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