Fix dynamic RoPE scaling formula

[dkuku]

In dynamic RoPE scaling, base was inadvertently raised to the power of dim / (dim - 2) along with the context scaling multiplier due to pipeline operator ordering.
According to NTK-aware dynamic scaling:

$$ \text{base}' = \text{base} \times \left( \frac{\alpha \times s}{s_{\text{orig}}} - (\alpha - 1) \right)^{\frac{d}{d-2}} $$

Previously, (base * scale) was raised to the exponent. This PR fixes the calculation so only the scaling factor is exponentiated before multiplying with base:

    # Previous (buggy): (base * scale) ^ (d / (d - 2))
    base
    |> Nx.multiply(scale)
    |> Nx.pow(size / (size - 2))

    # Fixed: base * (scale ^ (d / (d - 2)))
    Nx.multiply(base, Nx.pow(scale, size / (size - 2)))

References

• Matching implementation in Hugging Face modeling_rope_utils.py https://github.com/huggingface/transformers/blob/main/src/transformers/modeling_rope_utils.py (_compute_dynamic_ntk_parameters)
• Mathematical derivation: NTK-aware Scaling Context Extension https://mbrenndoerfer.com/writing/ntk-aware-scaling-context-extension
• Original proposal: Reddit /r/LocalLLaMA discussion https://www.reddit.com/r/LocalLLaMA/comments/14mrgpr/dynamically_scaled_rope_further_increases/

This PR fixes the calculation and adds a unit test in LayersTest.

codex and antigravity was used to find and fix it.

[jonatanklosko]

Good catch, thank you!