metal : optimize concat kernel and fix set kernel threads (llama/23411)

* metal : fix GGML_OP_SET kernel threads

* tests : extend test_cpy to support different src/dst shapes

Extend test_cpy to support different source and destination tensor shapes
for CPY operations (reshaping), where the total number of elements must match.

- Renamed ne -> ne_src, added ne_dst parameter (default: use src shape)
- Added 50 new reshaping test cases covering 1D<->2D<->3D<->4D conversions
- Tests exercise 1024 boundary, small shapes, and large dimensionality changes
- Fixed dangling reference bug (storing & to temporary std::array)
- Updated all existing test calls with permute/transpose args for compatibility

Assisted-by: llama.cpp:local pi

* metal : optimize concat kernel with row batching for small widths

When ne0 < 256, batch multiple rows into a single threadgroup to improve
occupancy. This avoids underutilizing the GPU when processing narrow tensors.

- Dispatch nth = min(256, ne0) threads per group
- Calculate nrptg (rows per threadgroup) to fill up to 256 threads
- Update kernel index calculation to handle the row batching
- Add boundary check for i1 >= ne1

Assisted-by: llama.cpp:local pi

* tests : clean-up

* tests : refactor CPY shape tests to use dimension permutations

Replace 75 hardcoded test cases with a loop over permutations of
{3, 5, 7, 32} (total elements: 3360). Each src permutation is tested
against canonical sorted and reverse dst, skipping identical shapes.
Covers F32, F16, and Q4_0 (when both src and dst ne0 == 32).

Assisted-by: llama.cpp:local pi

ggml/src/ggml-metal/ggml-metal-ops.cpp

@@ -564,9 +564,20 @@ int ggml_metal_op_concat(ggml_metal_op_t ctx, int idx) {
     ggml_metal_encoder_set_buffer  (enc, ggml_metal_get_buffer_id(op->src[1]), 2);
     ggml_metal_encoder_set_buffer  (enc, ggml_metal_get_buffer_id(op),         3);
 
-    const int nth = std::min(1024, ne0);
+    int nth = std::min(256, ne0);
 
-    ggml_metal_encoder_dispatch_threadgroups(enc, ne1, ne2, ne3, nth, 1, 1);
+    // when rows are small, we can batch them together in a single threadgroup
+    int nrptg = 1;
+    if (nth < 256) {
+        nrptg = std::min((256 + nth - 1) / nth, ne1);
+        if (nrptg * nth > 256) {
+            nrptg = 256 / nth;
+        }
+    }
+
+    const int nw0 = (ne1 + nrptg - 1) / nrptg;
+
+    ggml_metal_encoder_dispatch_threadgroups(enc, nw0, ne2, ne3, nth, nrptg, 1);
 
     return 1;
 }
@@ -1786,7 +1797,7 @@ int ggml_metal_op_set(ggml_metal_op_t ctx, int idx) {
         nk0 = ne10/ggml_blck_size(op->type);
     }
 
-    int nth = std::min<int>(nk0, ggml_metal_pipeline_max_theads_per_threadgroup(pipeline));
+    int nth = std::min<int>(nk0*ne11, 256);
 
     // when rows are small, we can batch them together in a single threadgroup
     int nrptg = 1;
@@ -1797,7 +1808,7 @@ int ggml_metal_op_set(ggml_metal_op_t ctx, int idx) {
             nrptg = (nth + nk0 - 1)/nk0;
             nth   = nk0;
 
-            if (nrptg*nth > ggml_metal_pipeline_max_theads_per_threadgroup(pipeline)) {
+            if (nrptg*nth > 256) {
                 nrptg--;
             }
         }

ggml/src/ggml-metal/ggml-metal.metal

@@ -7486,7 +7486,11 @@ kernel void kernel_concat(
 
     const int i3 = tgpig.z;
     const int i2 = tgpig.y;
-    const int i1 = tgpig.x;
+    const int i1 = ntg.y == 1 ? tgpig.x : tgpig.x*ntg.y + tpitg.y;
+
+    if (i1 >= args.ne1) {
+        return;
+    }
 
     int o[4] = {0, 0, 0, 0};
     o[args.dim] = args.dim == 0 ? args.ne00 : (args.dim == 1 ? args.ne01 : (args.dim == 2 ? args.ne02 : args.ne03));