Fix/transformers conversation import

[umihesh]

What this PR does

This PR lazily imports the Conversation class from transformers.pipelines and handles its absence gracefully.

Why

As of transformers>=4.48.0, the Conversation class has been removed. MLServer currently imports it unconditionally, causing an ImportError at startup - even if the user is not using conversational models.

This change:

• Moves the import inside a try-except block
• Skips Conversation logic if the class is unavailable

This allows MLServer to work with newer versions of transformers while maintaining backward compatibility.

Related

Fixes #1955

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}