feat: Add unroll pos docs script for biencoder training data

[shan-nvidia]

What does this PR do ?

Unroll biencoder training data with multiple positive documents into records with single positive documents.

Changelog

• Add a script that takes training data where each question has multiple positive documents and creates a new dataset where each (question, pos_doc) pair becomes its own record.
• The script is manually tested.
• The script has doc string as the documentation.

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[akoumpa]

/ok to test 2af51f7

[akoumpa]

/ok to test 054980a