[Relax][PyTorch] Fix PyTorch Dynamo frontend for Darwin compatibility

[guan404ming]

Why

The llvm_target() function reads /proc/cpuinfo which only exists on Linux, causing tests to fail on macOS with FileNotFoundError.

How

• Add cross-platform CPU feature detection in llvm_target() using platform.system() and sysctl for macOS
• Update tests

[gemini-code-assist]

Summary of Changes

Hello @guan404ming, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a critical compatibility issue in the PyTorch Dynamo frontend, specifically for macOS users. It refactors the CPU feature detection mechanism to be platform-aware, ensuring that the system can correctly identify and utilize available CPU instructions without encountering errors on non-Linux environments. This change broadens the usability of the PyTorch Dynamo frontend across different operating systems.

Highlights

• macOS Compatibility Fix: The llvm_target() function, which previously relied on /proc/cpuinfo (a Linux-specific file), now includes cross-platform CPU feature detection to prevent FileNotFoundError on macOS.
• Cross-Platform CPU Detection: Implemented platform-specific logic within llvm_target(): for Linux, it continues to read /proc/cpuinfo, while for macOS (Darwin), it utilizes sysctl -n machdep.cpu.features to determine CPU capabilities like AVX512 or AVX2. A fallback to 'llvm' is provided for ARM64 macOS and other cases.
• Test Updates: Existing tests for the PyTorch Dynamo frontend were updated to reflect changes in expected output, specifically regarding the order of elements in a returned tuple and constant types within subgraph_0, and parameter/variable naming consistency in subgraph_1.

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[gemini-code-assist]

Code Review

This pull request addresses a compatibility issue on macOS by making the llvm_target() function cross-platform. The changes correctly use platform.system() to differentiate between Linux and Darwin, and employ sysctl on macOS to detect CPU features. This resolves the FileNotFoundError for /proc/cpuinfo on non-Linux systems. The test updates also seem appropriate to align with the new logic. I have one suggestion to improve the maintainability and robustness of the llvm_target function.

[guan404ming]

cc @tlopex @mshr-h