[BC] Fixing usage of text pairs
Author: Narsil
Pull request: yes

# This contains a BC please read full description
# What does this PR do?

The BC is actually preventing users from misusing the pipeline since
users could have been willing to send text pairs and the pipeline would
instead understand the thing as a batch returning bogus results.

The correct usage of text pairs is preserved in this PR even when that
makes the code clunky.

Adds support for `{"text":..,, "text_pair": ...}` inputs for both dataset
iteration and more explicit usage to pairs.

Fixes #17305



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Fixes # (issue)


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Comments:
Pinging two persons, since this contains a backward breaking change (even if it was bugged usage) I would rather have more eyes than not here.

The linked issue contains more information that could be helpful too.

_The documentation is not available anymore as the PR was closed or merged._