iq1_s: use IQ2_XXS for attn_output

At a cost of 0.04 extra bpw this gives a big improvement in PPL.

Author: Iwan Kawrakow

llama.cpp

@@ -10112,6 +10112,9 @@ static ggml_type get_k_quant_type(quantize_state_internal & qs, ggml_type new_ty
             if (qs.i_ffn_down < qs.n_ffn_down/8) new_type = GGML_TYPE_Q2_K;
             ++qs.i_ffn_down;
         }
+        else if (name.find("attn_output.weight") != std::string::npos) {
+            if (ftype == LLAMA_FTYPE_MOSTLY_IQ1_S) new_type = GGML_TYPE_IQ2_XXS;
+        }
     } else if (name.find("attn_v.weight") != std::string::npos) {
         if      (ftype == LLAMA_FTYPE_MOSTLY_Q2_K) {
             new_type = qs.model.hparams.n_gqa() >= 4 ? GGML_TYPE_Q4_K : GGML_TYPE_Q3_K;