Make the model config override the pretrained config

[jlamypoirier]

🎯 Goal (What & Why)

Currently, a pretrained config overrides an arbitrary part of the user-specified config. This causes a lot of troubles:

• We typically override the architecture parameters only, which complicates things when also want to throw in some non-architecture parameters Missing configuration when converting from HF model config json #166
• Some default values are set before loading the pretrained configs and end up being wrong [bug] Inconsistent init_method_std in test_load_distributed_checkpoint_dp2 #88.
• Values set in the config are silently ignored and can be confusing. Ex the actual hidden size may end up being 4096 when the config explicitly says 2048.

I suggest flipping things around so the specified model config overrides the pretrained config. This should give us the behaviour we want in most cases:

• Pretrained config, no base model config: All architecture parameters are imported, and so are relevant non-architecture parameters (ex. window_size). Other non-architecture parameters take the Fast-LLM default.
• Pretrained config, base model config with non-architecture parameters: Parameters explicitly specified in the base model config are taken, others are as above.
• Pretrained config, base model config with architecture parameters: We probably want to enforce matching values, and raise an error for any mismatch. (This would be an improvement because right now wrong values are silently ignored.)
• No pretrained config: Same as before.

🚀 Execution Plan

We can use Fast-LLM's override mechanism as in #168.
However, we'll also need to adapt the update mechanism to get the behaviour we want for nested configs.
It could also be difficult to achieve backward compatibility.

📌 Acceptance Criteria (Must-Haves for Completion)

• Things should work as described above

🛠️ Project Management

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