metal : add TQ2_0 support (#26980)

* metal: add TQ2_0 support

Add support for the GGML_TYPE_TQ2_0 (ternary, 2 bits per element) type in
the Metal backend.

Assisted-by: llama.cpp:DeepSeek-v4-Flash-0731

* cont : optimize mul_mv kernel

- float ops over integer ops
- precalculate sums
- hoist coef out of the inner loop
- contiguous y loads

llama.cpp:DeepSeek-v4-Flash-0731

ggml/src/ggml-metal/ggml-metal-device.cpp

@@ -953,6 +953,11 @@ ggml_metal_pipeline_with_params ggml_metal_library_get_pipeline_mul_mv(ggml_meta
                 nr0 = N_R0_IQ4_XS;
                 smem = 32*sizeof(float);
             } break;
+        case GGML_TYPE_TQ2_0:
+            {
+                nsg = N_SG_TQ2_0;
+                nr0 = N_R0_TQ2_0;
+            } break;
         default:
             {
                 GGML_LOG_ERROR("Asserting on type %d\n", (int) tsrc0);
@@ -1182,6 +1187,11 @@ ggml_metal_pipeline_with_params ggml_metal_library_get_pipeline_mul_mv_id(ggml_m
                 nr0 = N_R0_IQ4_XS;
                 smem = 32*sizeof(float);
             } break;
+        case GGML_TYPE_TQ2_0:
+            {
+                nsg = N_SG_TQ2_0;
+                nr0 = N_R0_TQ2_0;
+            } break;
         default:
             {
                 GGML_LOG_ERROR("Asserting on type %d\n", (int)op->src[2]->type);