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

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[rlleshi]

@RobertSamoilescu kind reminder on this PR