Create down conversion for GroupNormalization-21

[justinchuby]

When converters are adopting the new operator in opset 21, there will be needs for converting to lower opsets when users are trying to use runtimes that have not caught up with support.

The converter will help us adopt the latest opset without having to wait for all runtimes to catch up.

We don’t need to use the same operator (GroupNorm) when converting back, as long as the model is correct and accurate.

It is possible to use InstanceNorm and some transpose for it. For example https://github.com/microsoft/onnxscript/blob/172a81745c10467e04d9b4fdfa7829ce3321b6ce/onnxscript/function_libs/torch_lib/ops/core.py#L5850