imageUtilKernels.cu: optimize initAttentionMaskKernel

[vc-zju]

What does this PR do?

Type of change: Optimization

Overview:
Simply change the loop order in imageUtilKernels.cu:initAttentionMaskKernel. Take Qwen2.5-VL as example, optimize the kernel from 2.8ms to 0.4ms for 640*640 image input.
Before

After

Testing

Just use nsys to profile it and you can see it.

• Make sure you read and follow Contributor guidelines and your commits are signed.
• Is this change backward compatible?: Yes
• Did you write any new necessary tests?: Just reuse previous test
• Did you add or update any necessary documentation?: No need to do that
• Did you update Changelog?: No

[fans-nv]

Would you mind sharing on which platform did you collect the performance data?
And very nice catch for the perf issue, thanks for the contribution.

[fans-nv]

Similar to to other community PRs, I need to cherry-pick to internal branch and then publish the change to main along with next release.

[vc-zju]

Sure, I collect it on jetson-Thor. I can also move this to internal gitlab if you think we need to do that.