tensor-split meta backend fixes (#26502)

* backend: propagate buffer usage in meta backend

* ggml-meta: make sure to call init_tensor for all new tensors

* meta: remove explicit check for meta backend in ggml_backend_meta_get_split_state

I can't seem to reproduce the original failure in the latest code.

ggml/src/ggml-backend.cpp

@@ -182,6 +182,8 @@ void ggml_backend_buffer_set_usage(ggml_backend_buffer_t buffer, enum ggml_backe
     // FIXME: add a generic callback to the buffer interface
     if (ggml_backend_buffer_is_multi_buffer(buffer)) {
         ggml_backend_multi_buffer_set_usage(buffer, usage);
+    } else if (ggml_backend_buffer_is_meta(buffer)) {
+        ggml_backend_meta_buffer_set_usage(buffer, usage);
     }
 }