ggml: Seperate tensor load from backend creation

Currently, when the backend is created, the tensors are loaded at the
same time, which is a slow operation. This separates them to be two
steps:
 - Create backend, including enumerating tensors and memory allocation
 - Loading tensor data

This allows more flexibility in managing model loading.

fs/ggml/gguf_test.go

@@ -35,7 +35,7 @@ func TestWriteGGUF(t *testing.T) {
 	}
 	defer r.Close()
 
-	ff, _, err := Decode(r, 0)
+	ff, err := Decode(r, 0)
 	if err != nil {
 		t.Fatal(err)
 	}