Fix chat prompt in HFAgent
# What does this PR do?

There was a bug in formatting prompts in the chat mode. Actually, user-provided custom prompts were never used.

## Who can review?

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.

@sgugger, please review this PR
_The documentation is not available anymore as the PR was closed or merged._