Fix yaml serialization in io mixin
> Add docstring

Updated in https://github.com/NVIDIA/NeMo/pull/11106/commits/bd903a0c86b8ba847903f9ba8e1aad37f1f5ea6a [🤖]: Hi @hemildesai 👋,

We wanted to let you know that a [CICD pipeline](https://github.com/NVIDIA/NeMo/actions/runs/11602055172) for this PR just finished successfully

So it might be time to merge this PR or get some approvals

I'm just a bot so I'll leave it you what to do next.

//cc @pablo-garay @ko3n1g CI passed : https://github.com/NVIDIA/NeMo/actions/runs/11602055172 There was a further commit after the triggered CI run but it was a docstring name + name change which shouldnt affect result:
https://github.com/NVIDIA/NeMo/pull/11106/commits/bd903a0c86b8ba847903f9ba8e1aad37f1f5ea6a

Uses __class__ only when data.__qualname__ or similar fails