fix: Ensure activation scaling factor set before initializing b_dec

[keltin13]

Description

Fixes #439

When b_dec_init_method is set to mean or geometric_median we require getting activations from the buffer, however set_norm_scaling_factor_if_needed was not being called prior to this, meaning if normalize_activations was set to expected_average_only_in it would throw an error.

I added a call to set_norm_scaling_factor_if_needed directly before accessing the activations buffer inside _init_sae_group_b_decs to ensure the scaling factor is set before initialization. I also added a condition to set_norm_scaling_factor_if_needed to check whether the scaling factor is already set to prevent it from being set twice.

An alternative solution would be to take this opportunity to resolve the TODO for _init_sae_group_b_decs and move that method to another class, which I'm happy to do as well.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
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• I have added tests that prove my fix is effective or that my feature works
• New and existing tests pass locally with my changes
• I have not rewritten tests relating to key interfaces which would affect backward compatibility

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