allow masking on consecutive messages with same roles

[lsy641]

What does this PR do?

Fixes # (issue)

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• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Was this discussed/approved via a GitHub issue? Please add a link
  to it if that's the case.
  Correct masking when the same roles are present in adjacent messages in DataCollatorForCompletionOnlyLM  #1994
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• Did you write any new necessary tests?

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