Fix Transformers backend compatibility check

[hmellor]

Prerequisites:

• Upgrade transformers to v4.50.0 #13905

Explanation:

• is_backend_compatible is a method that all PreTrainedModel will have
• flex attention is an arbitratry attention interface that shouldn't determine compatibility

cc @ArthurZucker

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[ArthurZucker]

lgtm, I don't remember if is_backend_compatible was in 4.49 or not... time flies

[hmellor]

Ah good point, looks like this is another one to add to the stack on top of #13905