rope_theta is ignored on the dense attention path (hardcoded 10000 / 1000000)

Start with the dense timescale construction in activations.h and CreateInvTimescale in ops/ops.h, then inspect ModelConfig serialization in configs.h and python/configs.cc. Check attention.cc to understand global-table selection. Done means the configured local and global theta values are serialized and used without changing existing outputs; verify against the existing goldens.

ModelConfig::rope_theta (configs.h:852) is read in exactly one place — the mla_* timescales in struct Activations (activations.h:512-520), i.e. the MLA path. The dense path builds its two tables from constants instead:

inv_timescale(
    CreateInvTimescale(allocator, layer_config.qkv_dim,
                       layer_config.post_qk == PostQKType::HalfRope)),
inv_timescale_global(
    CreateInvTimescale(allocator, max_qkv_dim,
                       layer_config.post_qk == PostQKType::HalfRope,
                       1000000.0, config.partial_rotary_factor)) {

(activations.h:126-132 — the 10000.0 for the local table comes from the default argument at ops/ops.h:30.)

Nothing is wrong today. Gemma 2 / PaliGemma / T5Gemma / Gemma3-270M want 10000 on every layer; Gemma 3 and Gemma 4 want 10000 local + 1e6 global; Qwen3 wants 1e6 and reaches it via use_global_timescale plus all-global windows. Every model in the tree matches one of the two constants, so this is latent rather than a live bug.

What concerns me is the failure mode for the next model. rope_theta looks like the knob for this, silently isn't on the dense path, and a mismatch produces no assert and no warning — just quietly degraded output that doesn't look like a position-encoding problem. A Llama-style 500000, or any future Gemma with different values, would run at 10000/1e6 and merely seem "worse than expected."

Proposed fix

1. Pass config.rope_theta to the local CreateInvTimescale call (config is already a ctor parameter, so no new plumbing).
2. Add ModelConfig::global_rope_theta = 1000000.0f, appended at the end of VisitFields for serialization compatibility, plus the matching entry in python/configs.cc; pass it to the global call.
3. Optionally drop the base_frequency default from CreateInvTimescale so every call site has to state its theta.
4. Set both fields explicitly in the Qwen3 configs, so their correctness is stated rather than incidental.

This is behavior-preserving: the only assignment of rope_theta anywhere is DeepSeek's = 10000.0f (configs.cc:672), which equals the default, and the new field's default equals the current literal — so every config in the tree produces bit-identical timescales. It should be checkable against existing goldens.

One design question

After this, use_global_timescale is arguably redundant: the selector at attention.cc:156 could use the global table whenever global_rope_theta != rope_theta. That removes a flag you can forget to set — setting only global_rope_theta today silently does nothing — but it touches the 10 configs that set it. Happy to keep the flag or drop it, whichever you prefer.

Glad to send a PR if this looks right.

(Separately, and I can file it on its own if it's worth a look: partial_rotary_factor is passed only to the global table, so on Gemma 4 2B — where 4 of every 5 layers are local — the local layers get full rotary. I couldn't tell from the code whether that asymmetry is intentional.)