Stamp weight dtype on GQA PackQKV Concat/Transpose intermediates

[Copilot]

The PackQKVForGQA / PackQKVWithBiasForGQA rewrites build the packed weight with op.Concat / op.Transpose, whose output values carry no declared type. When those intermediates are later folded into initializers there is no declared dtype to inherit, so an fp16 model could end up with fp32 packed weights — the drop that #351 mitigated downstream.

The issue asked whether this still reproduces. It does. On main, a CPU-EP llama build gives:

Concat    node_Concat_0    [FLOAT, FLOAT, FLOAT] -> None
Transpose node_Transpose_1 [None]                -> None

Changes

• _group_query_attention.py — new _propagate_dtype(source, *targets) helper resolves the source parameter's dtype through the existing initializer_dtype() (declared type, const_value fallback, fail-closed on mismatch) and stamps it on the rewrite-created values.
• PackQKVForGQA.rewrite — stamps packed_w / packed_wt from q_w.
• PackQKVWithBiasForGQA.rewrite — same, plus packed_bias from bias_q. The bias Concat folds into an initializer too and had the identical untyped-output problem, so it is covered here rather than left for a follow-up.
• Tests — parameterized fp32/fp16 regression tests for both rewrite paths, asserting the Concat/Transpose/bias-Concat outputs declare the model dtype. They fail on unmodified source (assert None == FLOAT16).

packed_w = op.Concat(q_w, k_w, v_w, axis=0)
packed_wt = op.Transpose(packed_w, perm=[1, 0])
# Concat/Transpose outputs have no declared type; carry the weight dtype
# forward so the folded packed initializer keeps the model dtype.
_propagate_dtype(q_w, packed_w, packed_wt)

The #351 fold-pass stamping and const_value fallback are left untouched as defense-in-depth.

Note: the onnxruntime_easy collection blocker cited as the reason this was deferred from #351 no longer applies — it ships in the testing extra, so src/mobius/rewrite_rules/_group_query_attention_test.py runs directly.

• Fixes GQA PackQKV rewrite leaves Concat/Transpose intermediates without a declared dtype #355

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