Make PipelineParallelLayout always return str from __repr__

[ananthsub]

What does this PR do ?

From https://github.com/NVIDIA-NeMo/Megatron-Bridge/pull/1133/files#r2479292501
This avoids serialization issues when dumping the TransformerConfig to W&B since PipelineParallelLayout.__repr__ can return a list instead of a str, depending on the input data type

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[ananthsub]

/ok to test 370a795

[ananthsub]

/ok to test 2a13a61