rpc : better caching of the base buffer pointer

There is no need to use map, just store the base pointer in the buffer
context.

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

@@ -181,7 +181,7 @@ struct ggml_backend_rpc_context {
 
 struct ggml_backend_rpc_buffer_context {
     std::shared_ptr<socket_t> sock;
-    std::unordered_map<ggml_backend_buffer_t, void *> base_cache;
+    void * base_ptr;
     uint64_t remote_ptr;
 };
 
@@ -423,16 +423,15 @@ static void ggml_backend_rpc_buffer_free_buffer(ggml_backend_buffer_t buffer) {
 
 static void * ggml_backend_rpc_buffer_get_base(ggml_backend_buffer_t buffer) {
     ggml_backend_rpc_buffer_context * ctx = (ggml_backend_rpc_buffer_context *)buffer->context;
-    if (ctx->base_cache.find(buffer) != ctx->base_cache.end()) {
-        return ctx->base_cache[buffer];
+    if (ctx->base_ptr != nullptr) {
+        return ctx->base_ptr;
     }
     rpc_msg_buffer_get_base_req request = {ctx->remote_ptr};
     rpc_msg_buffer_get_base_rsp response;
     bool status = send_rpc_cmd(ctx->sock, RPC_CMD_BUFFER_GET_BASE, &request, sizeof(request), &response, sizeof(response));
     GGML_ASSERT(status);
-    void * base_ptr = reinterpret_cast<void *>(response.base_ptr);
-    ctx->base_cache[buffer] = base_ptr;
-    return base_ptr;
+    ctx->base_ptr = reinterpret_cast<void *>(response.base_ptr);
+    return ctx->base_ptr;
 }
 
 static rpc_tensor serialize_tensor(const ggml_tensor * tensor) {
@@ -557,7 +556,7 @@ static ggml_backend_buffer_t ggml_backend_rpc_buffer_type_alloc_buffer(ggml_back
     if (response.remote_ptr != 0) {
         ggml_backend_buffer_t buffer = ggml_backend_buffer_init(buft,
             ggml_backend_rpc_buffer_interface,
-            new ggml_backend_rpc_buffer_context{sock, {}, response.remote_ptr},
+            new ggml_backend_rpc_buffer_context{sock, nullptr, response.remote_ptr},
             response.remote_size);
         return buffer;
     } else {