rpc : keep last_graph_uid in the device context (ggml-org#23273)

With the introduction of MTP we can have multiple compute contexts for
the same RPC device. In this case last_graph_uid is not updated properly
when contexts are being switched. This patch fixes this by moving
last_graph_uid to the device context, making sure it is always updated.

closes: ggml-org#23242

Author: rgerganov

ggml/src/ggml-rpc/ggml-rpc.cpp

@@ -199,6 +199,14 @@ static ggml_guid_t ggml_backend_rpc_guid() {
     return &guid;
 }
 
+struct ggml_backend_rpc_device_context {
+    std::string endpoint;
+    uint32_t    device;
+    std::string name;
+    std::string description;
+    uint64_t    last_graph_uid;
+};
+
 struct ggml_backend_rpc_buffer_type_context {
     std::string endpoint;
     uint32_t    device;
@@ -211,7 +219,6 @@ struct ggml_backend_rpc_context {
     std::string endpoint;
     uint32_t    device;
     std::string name;
-    uint64_t    last_graph_uid;
 };
 
 struct ggml_backend_rpc_buffer_context {
@@ -691,17 +698,19 @@ static void serialize_graph(uint32_t device, const ggml_cgraph * cgraph, std::ve
 
 static enum ggml_status ggml_backend_rpc_graph_compute(ggml_backend_t backend, ggml_cgraph * cgraph) {
     ggml_backend_rpc_context * rpc_ctx = (ggml_backend_rpc_context *)backend->context;
+    ggml_backend_dev_t rpc_dev = ggml_backend_get_device(backend);
+    ggml_backend_rpc_device_context * rpc_dev_ctx = (ggml_backend_rpc_device_context *)rpc_dev->context;
 
     GGML_ASSERT(cgraph->n_nodes > 0);
-    bool reuse = cgraph->uid != 0 && rpc_ctx->last_graph_uid == cgraph->uid;
+    bool reuse = cgraph->uid != 0 && rpc_dev_ctx->last_graph_uid == cgraph->uid;
     if (reuse) {
         rpc_msg_graph_recompute_req request;
         request.device = rpc_ctx->device;
         auto sock = get_socket(rpc_ctx->endpoint);
         bool status = send_rpc_cmd(sock, RPC_CMD_GRAPH_RECOMPUTE, &request, sizeof(request));
         RPC_STATUS_ASSERT(status);
     } else {
-        rpc_ctx->last_graph_uid = cgraph->uid;
+        rpc_dev_ctx->last_graph_uid = cgraph->uid;
         std::vector<uint8_t> input;
         serialize_graph(rpc_ctx->device, cgraph, input);
         auto sock = get_socket(rpc_ctx->endpoint);
@@ -770,7 +779,6 @@ ggml_backend_t ggml_backend_rpc_init(const char * endpoint, uint32_t device) {
         /* .endpoint       = */ endpoint,
         /* .device         = */ device,
         /* .name           = */ dev_name,
-        /* .last_graph_uid = */ 0,
     };
     auto reg = ggml_backend_rpc_add_server(endpoint);
     ggml_backend_t backend = new ggml_backend {
@@ -1757,15 +1765,6 @@ void ggml_backend_rpc_start_server(const char * endpoint, const char * cache_dir
     }
 }
 
-// device interface
-
-struct ggml_backend_rpc_device_context {
-    std::string endpoint;
-    uint32_t    device;
-    std::string name;
-    std::string description;
-};
-
 static const char * ggml_backend_rpc_device_get_name(ggml_backend_dev_t dev) {
     ggml_backend_rpc_device_context * ctx = (ggml_backend_rpc_device_context *)dev->context;
 
@@ -1947,10 +1946,11 @@ ggml_backend_reg_t ggml_backend_rpc_add_server(const char * endpoint) {
         std::string dev_name = "RPC" + std::to_string(dev_id);
         std::string dev_desc = std::string(endpoint);
         ggml_backend_rpc_device_context * dev_ctx = new ggml_backend_rpc_device_context {
-            /* .endpoint    = */ endpoint,
-            /* .device      = */ ind,
-            /* .name        = */ dev_name,
-            /* .description = */ dev_desc
+            /* .endpoint    = */    endpoint,
+            /* .device      = */    ind,
+            /* .name        = */    dev_name,
+            /* .description = */    dev_desc,
+            /* .last_graph_uid = */ 0,
         };
 
         ggml_backend_dev_t dev = new ggml_backend_device {