Remove dead code: unused _is_blackwell() in attention/utils.py

[Ashutosh0x]

Summary

Remove dead code: _is_blackwell() in torchao/prototype/attention/utils.py is defined but never imported or called anywhere in the codebase.

Verified by searching the entire repository -- only one hit exists (the definition itself at line 19). No callers, imports, or references.

Partial fix for #4333 (item #7)

Test Plan

Get-ChildItem -Path torchao -Recurse -Include *.py |
  Select-String -Pattern "_is_blackwell" |
  Select-Object Path, LineNumber
# Result: only utils.py:19 (the definition)

This change was authored with the assistance of an AI coding assistant.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/ao/4501

• 📄 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, 2 Unrelated Failures

As of commit cd518c1 with merge base 5647df9 ():

NEW FAILURES - The following jobs have failed:

• Code Analysis with Ruff / build (3.9) (gh)
  Process completed with exit code 1.
• PR Label Check / Check PR Labels (gh)
  Process completed with exit code 1.

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• Run Regression Tests / test (CPU 2.12, linux.4xlarge, torch==2.12.0 torchvision==0.27.0 --index-url https://download.pyt... / linux-job (gh) (trunk failure)
  test/quantization/pt2e/test_x86inductor_fusion.py::DynamicShapesCppWrapperCpuTests::test_qconv2d_maxpool2d_linear_dynamic_cpu_dynamic_shapes_cpp_wrapper
• Run Regression Tests / test (CUDA 2.12, linux.g5.12xlarge.nvidia.gpu, torch==2.12.0 torchvision==0.27.0 --index-url http... / linux-job (gh) (trunk failure)
  test/quantization/pt2e/test_x86inductor_fusion.py::DynamicShapesCppWrapperCpuTests::test_qconv2d_maxpool2d_linear_dynamic_cpu_dynamic_shapes_cpp_wrapper

This comment was automatically generated by Dr. CI and updates every 15 minutes.