[Proposal] Hybrid architectures warn on unresolved optional 'hook_attn' aliases when they are skipped #1578

Proposal

When setup_submodules skips an optional submodule, prune that block’s hook_aliases whose target path starts at the skipped key (e.g. drop hook_attn_out → attn.hook_out when attn was skipped). Copy the aliases dict per layer so templates don’t share mutations; leave residual/MLP aliases alone. Full-attention siblings keep HT aliases; GDN native aliases are unchanged. Cover with unit tests for prune + no warning on register.

Motivation

When loading models like Qwen-3.5 Hybrid, a warning is printed per hybrid layer, cluttering the terminal quickly (as noted in #1547). The proposed solution would remove unused aliases since it is expected behavior for GDN layers to use their native hook_linear_attn_out instead.

UserWarning: Hook alias 'hook_attn_out' -> 'attn.hook_out' on BlockBridge(name='model.language_model.layers.2') did not resolve; this hook will not be accessible.
  getattr(module, "_register_aliases")()

Pitch

Loading hybrids like Qwen3.5 spam harmless hook_attn_out did not resolve warnings on every GDN/Mamba-2 layer. Those layers never had full attn; they expose hook_linear_attn_out (and similar) instead. Treat that as expected, not a fault, so boot stays quiet without hiding real alias bugs on full-attention layers.

Alternatives

The warning could be left as-is, as this is not a functional error and the user might want to be notified at build-time that the standard hook_attn_out alias will be missing, though it could be caught and reworded to reflect its non-breaking nature.

Checklist

• I have checked that there is no similar issue in the repo (required)