PyTorch: [Docs] Add accelerator backend test reuse guide (#189376)

Executive Summary

Summary This PR adds a guideline document for out-of-tree (OOT) accelerator backend developers on reusing PyTorch's upstream test suite. The guide covers: Test categorization and the role of hardware classification in selecting applicable tests. Test reuse strategies for OOT backends, including adapting upstream tests to match backend capabilities without modifying upstream tests.

Hyperscale Open Source & Cloud Infrastructure Review

From an infrastructure engineering, cloud-native scale, and hyperscale governance standpoint: - **Hyperscale Provenance:** Open-sourcing internal frameworks subjects proprietary systems to rigorous public analysis. - **De-facto Standard Cohesion:** Publishing enterprise tooling establishes interoperable specifications across distributed execution. - **Vendor Decoupling:** Platform engineers can inspect underlying telemetry and memory layouts without black-box vendor lock-in.

Impact on the Open Ecosystem

Bridges enterprise engineering scale with independent, reproducible open-source software stacks.