metal : add opt_step_adamw and op_sum (#16529)

* scaffold to support opt step adamw on metal (not written so far)

* add opt-step-adamw kernel for metal

* pass op->src[4] as a separate buffer to the pipeline

* add bounds check to opt-step-adamw kernel

* complete scaffold for GGML_OP_SUM

* naive GGML_OP_SUM kernel

* remove unwanted comment

* change OP_SUM capability gate

* Add has_simdgroup_reduction to both ops to pass CI

ggml/src/ggml-metal/ggml-metal-device.m

@@ -656,6 +656,7 @@ bool ggml_metal_device_supports_op(ggml_metal_device_t dev, const struct ggml_te
         case GGML_OP_COS:
         case GGML_OP_LOG:
             return ggml_is_contiguous(op->src[0]) && op->src[0]->type == GGML_TYPE_F32;
+        case GGML_OP_SUM:
         case GGML_OP_SUM_ROWS:
         case GGML_OP_MEAN:
         case GGML_OP_SOFT_MAX:
@@ -798,6 +799,8 @@ bool ggml_metal_device_supports_op(ggml_metal_device_t dev, const struct ggml_te
                         return false;
                 };
             }
+        case GGML_OP_OPT_STEP_ADAMW:
+            return has_simdgroup_reduction;
         default:
             return false;
     }