Remove redundant CUDA copies after gated_delta_net. (#23940)

* Remove redundant CUDA copies after gated_delta_net.

Currently, GDN writes recurrent state snapshots into its output tail, then the graph immediately copies those snapshots into ssm_states_all. With MTP draft length 3, target decode uses K=4, so that becomes 4 extra ggml_cuda_cpy calls.

The change detects that gated_delta_net -> view -> cpy pattern and makes the CUDA GDN kernel write the state snapshot(s) directly into the recurrent cache, skipping the intermediate tail writes and copy kernels when safe.

* Address review comments

ggml/src/ggml-cuda/gated_delta_net.cuh

@@ -1,4 +1,14 @@
 #include "common.cuh"
 #include "ggml.h"
 
+// fused-kernel recurrent-state output; strides in elements (per-seq stride is always D, set in-kernel)
+struct ggml_cuda_gated_delta_net_fused_cache {
+    float * data;        // rollback slot 0
+    int64_t slot_stride; // between rollback slots (0 when K==1)
+};
+
 void ggml_cuda_op_gated_delta_net(ggml_backend_cuda_context & ctx, ggml_tensor * dst);
+
+// same op, but writes the snapshot(s) into the cache instead of dst (see ggml_cuda_try_gdn_cache_fusion)
+void ggml_cuda_op_gated_delta_net_fused_cache(ggml_backend_cuda_context & ctx, ggml_tensor * dst,
+                                              ggml_cuda_gated_delta_net_fused_cache cache);