rpc : reuse compute graph buffers (#21299)

Reuse the buffer for the ggml context which is used for creating the
compute graph on the server side. This partially addresses a memory leak
created by the CUDA backend due to using buffer addresses as cache
keys.

ref: #21265
ref: #20315

Author: rgerganov

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

@@ -1009,8 +1009,8 @@ class rpc_server {
     bool get_device_memory(const rpc_msg_get_device_memory_req & request, rpc_msg_get_device_memory_rsp & response);
 
     struct stored_graph {
-        ggml_context_ptr ctx_ptr;
-        ggml_cgraph *    graph;
+        std::vector<uint8_t>   buffer;
+        ggml_cgraph          * graph;
     };
 
 private:
@@ -1518,10 +1518,12 @@ bool rpc_server::graph_compute(const std::vector<uint8_t> & input) {
     LOG_DBG("[%s] device: %u, n_nodes: %u, n_tensors: %u\n", __func__, device, n_nodes, n_tensors);
 
     size_t buf_size = ggml_tensor_overhead()*(n_nodes + n_tensors) + ggml_graph_overhead_custom(n_nodes, false);
-
+    if (stored_graphs[device].buffer.size() < buf_size) {
+        stored_graphs[device].buffer.resize(buf_size);
+    }
     struct ggml_init_params params = {
         /*.mem_size   =*/ buf_size,
-        /*.mem_buffer =*/ NULL,
+        /*.mem_buffer =*/ stored_graphs[device].buffer.data(),
         /*.no_alloc   =*/ true,
     };
     ggml_context_ptr ctx_ptr { ggml_init(params) };
@@ -1551,7 +1553,6 @@ bool rpc_server::graph_compute(const std::vector<uint8_t> & input) {
     }
     ggml_status status = ggml_backend_graph_compute(backends[device], graph);
     GGML_ASSERT(status == GGML_STATUS_SUCCESS && "Unsuccessful graph computations are not supported with RPC");
-    stored_graphs[device].ctx_ptr.swap(ctx_ptr);
     stored_graphs[device].graph = graph;
     return true;
 }