Add FlexAttention examples to SDPA tutorial

[dorbittonn]

Summary

• Add new section demonstrating flex_attention from PyTorch 2.5
• Include score_mod examples: relative position bias and ALiBi
• Show block_mask for sparse attention patterns (causal masking)
• Demonstrate combining score_mod and block_mask
• Add performance comparison with standard SDPA

Test plan

• All FlexAttention code tested with PyTorch 2.7.1
• Verified output shapes are correct
• Compilation with torch.compile works as expected

Checklist

• The issue that is being fixed is referred in the description
• Only one issue is addressed in this pull request
• Labels from the issue that this PR is fixing are added to this pull request
• No unnecessary issues are included into this pull request

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/tutorials/3674

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

❌ 2 New Failures

As of commit 6007d0f with merge base 7f8b6dc ():

NEW FAILURES - The following jobs have failed:

• Build tutorials (nightly/test) / build / pytorch_tutorial_build_worker (14, 15, linux.g5.4xlarge.nvidia.gpu) (gh)
  Process completed with exit code 2.
• link check on PR / linkChecker (gh)
  Process completed with exit code 1.

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