Fix incorrect dimension naming in Gemma3 projector
Author: kimwj94
Pull request: yes

# What does this PR do?

This PR fixes a misleading variable naming in `Gemma3MultiModalProjector.forward`.  
Currently, the third dimension of the vision encoder output (which represents the **hidden size**) is assigned to a variable named `seq_length`. This is confusing because the second dimension is the actual sequence length (number of patches).  
I have renamed `seq_length` to `hidden_size` to correctly reflect the tensor shape $(Batch, Sequence, Hidden)$ and to improve overall code readability. This is a non-functional change (NFC).


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## Who can review?

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@molbap
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Comments:
This looks correct to me but please run `make fix-repo` to propagate the changes in the modular file!

**[For maintainers]** Suggested jobs to run (before merge)

run-slow: gemma3, t5gemma2

@Rocketknight1 Thanks for the review! I've run `make fix-repo` and pushed the updates. The modular files should be synced now.

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