build: Update C++ standard from c++17 to c++20

[mc-nv]

What does the PR do?

• Upstream PyTorch raised ATen's minimum required C++ standard from 17 to 20 ([12/12] Enforce C++20 minimum in header guards (#178150) pytorch/pytorch#178150), breaking the pytorch_backend build.
• Bumps this repo's C++ standard default from 17 to 20 for consistency across the Triton stack.

Checklist

• PR title reflects the change and is of format <commit_type>: <Title>
• Changes are described in the pull request.
• Related issues are referenced.
• Populated github labels field
• Added test plan and verified test passes.
• Verified that the PR passes existing CI.
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• Added succinct git squash message before merging ref.
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Commit Type:

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• build
• ci
• docs
• feat
• fix
• perf
• refactor
• revert
• style
• test

Related PRs:

• build: Update C++ standard from c++17 to c++20 pytorch_backend#210
• build: Update C++ standard from c++17 to c++20 backend#129
• build: Update C++ standard from c++17 to c++20 checksum_repository_agent#16
• build: Update C++ standard from c++17 to c++20 identity_backend#38
• build: Update C++ standard from c++17 to c++20 local_cache#18
• build: Update C++ standard from c++17 to c++20 openvino_backend#123
• build: Update C++ standard from c++17 to c++20 redis_cache#26
• build: Update C++ standard from c++17 to c++20 repeat_backend#21
• build: Update C++ standard from c++17 to c++20 square_backend#24
• build: Update C++ standard from c++17 to c++20 tensorrt_backend#128
• build: Update C++ standard from c++17 to c++20 third_party#82
• build: Update C++ standard from c++17 to c++20 common#166
• build: Update C++ standard from c++17 to c++20 core#525
• build: Update C++ standard from c++17 to c++20 client#922
• build: Update C++ standard from c++17 to c++20 onnxruntime_backend#359
• build: Update C++ standard from c++17 to c++20 server#8962
• build: Update C++ standard from c++17 to c++20 python_backend#455
• build: Update C++ standard from c++17 to c++20 dali_backend#311

Where should the reviewer start?

• CMakeLists.txt - test/CMakeLists.txt

Test plan:

• CI Pipeline ID: 67062220

Caveats:

This repo has no direct dependency on ATen/libtorch; the bump is for cross-repo C++ standard consistency, not a confirmed build break here.

Background

Part of a coordinated C++17->C++20 bump across ~18 Triton component repos (TRI-1877), triggered by upstream PyTorch's ATen.h now requiring C++20.

Related Issues: (use one of the action keywords Closes / Fixes / Resolves / Relates to)

• Resolves: TRI-1877