ci: Update training script paths in BERT and T5

[balasaajay]

• I, the PR author, have personally reviewed every line of this PR.

What does this PR do ?

Update training script paths in BERT and T5 YAML configurations to reflect new directory structure

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[balasaajay]

/ok to test bd96e70

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/26311803502

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/26312658045