Make sure chat template isn't lost when truncating prompt.

[pramodith]

What does this PR do?

Skipping special tokens when truncating the prompt leads to the prompt no longer being in a valid chat template, if the input dataset is conversational.

This PR disables skipping special tokens.

Fixes #3644

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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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Who can review?

@LeonEricsson here's the quick fix.

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

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[LeonEricsson]

I've run some tests and this should be sufficient for now. However as you rightfully point out in #3646, the current truncation is too naive and will butcher the chat template. This needs to be addressed