fix(attention_visualizer): add default value for image_seq_length

[IceGiraffe]

What does this PR do?

Fixes a small bug in attention_visualizer.py where image_seq_length may be used before assignment.

🐛 Bug Description

Calling visualize_attention_mask() triggers:
UnboundLocalError: cannot access local variable 'image_seq_length' where it is not associated with a value

Fix

Just adds one line code

image_seq_length = None

Fixes #38576

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Who can review?

@ArthurZucker

[Rocketknight1]

Yes, this looks like a clean fix to me. Thank you!

[HuggingFaceDocBuilderDev]

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