feat: first cut at actual token generation and model loading

metal/set_rows.metal

@@ -0,0 +1,55 @@
+// DS4 Metal set-rows kernel used for KV writes.
+
+struct ds4_metal_args_set_rows {
+    int32_t  nk0;
+    int32_t  ne01;
+    uint64_t nb01;
+    uint64_t nb02;
+    uint64_t nb03;
+    int32_t  ne11;
+    int32_t  ne12;
+    uint64_t nb10;
+    uint64_t nb11;
+    uint64_t nb12;
+    uint64_t nb1;
+    uint64_t nb2;
+    uint64_t nb3;
+};
+
+// Scatters rows into the KV cache by token position. DS4 uses this after Q/K/V
+// preparation so decode and later prefill chunks can attend to previous tokens.
+template<typename T, typename TI>
+kernel void kernel_set_rows_f(
+        constant ds4_metal_args_set_rows & args,
+        device const char  * src0,
+        device const char  * src1,
+        device       float * dst,
+        uint3                tgpig[[threadgroup_position_in_grid]],
+        uint                 tiitg[[thread_index_in_threadgroup]],
+        uint3                tptg [[threads_per_threadgroup]]) {
+    const int32_t i03 = tgpig.z;
+    const int32_t i02 = tgpig.y;
+
+    const int32_t i12 = i03%args.ne12;
+    const int32_t i11 = i02%args.ne11;
+
+    const int32_t i01 = tgpig.x*tptg.y + tiitg/tptg.x;
+    if (i01 >= args.ne01) {
+        return;
+    }
+
+    const int32_t i10 = i01;
+    const TI      i1  = ((const device TI *) (src1 + i10*args.nb10 + i11*args.nb11 + i12*args.nb12))[0];
+
+          device T     * dst_row = (      device T     *) ((device char *) dst  + i1*args.nb1   + i02*args.nb2  + i03*args.nb3);
+    const device float * src_row = (const device float *) (                src0 + i01*args.nb01 + i02*args.nb02 + i03*args.nb03);
+
+    for (int ind = tiitg%tptg.x; ind < args.nk0; ind += tptg.x) {
+        dst_row[ind] = (T) src_row[ind];
+    }
+}
+
+typedef decltype(kernel_set_rows_f<float, int64_t>) set_rows_f_t;
+
+// Host-visible F32/I32 scatter variant used by KV-cache writes.
+template [[host_name("kernel_set_rows_f32_i32")]] kernel set_rows_f_t kernel_set_rows_f<float, int32_t>;