Custom TypeHandler and "No per-process OCDBT checkpoint subdirs" warning

Hello,

I've recently created a custom type handler.
Using it, and running on a single process I see the following warning

```python
WARNING:absl:[process=0][thread=async_save_18] Skipping merge of OCDBT checkpoints: No per-process OCDBT checkpoint subdirs found in /tmp/ckp3/115.orbax-checkpoint-tmp-136/callbacks.orbax-checkpoint-tmp-139,
```

The custom type handler I wrote serialises some custom type containing some numpy arrays, and if I was to run this across multiple processes I'd like only the master process to serialise the data (which is basically replicated).

How can I silence this warning? Did I forgot to define something?