refactor: enable multi-select dropdowns for platform and backend in bug_report.yml

[Ziminli]

Summary

This PR enables multi-select dropdowns for the platform and backend fields in .github/ISSUE_TEMPLATE/bug_report.yml.

Changes

• Enable Multi-Select Dropdown
  • Enables multi-select dropdowns for the platform and backend fields in .github/ISSUE_TEMPLATE/bug_report.yml by adding multiple: true.

Platform and Backend Affected

Platform

• N/A- CPU
• N/A- NVIDIA GPU
• N/A- MetaX GPU
• N/A- Cambricon MLU

Backend

• N/A- OpenMPI
• N/A- MPICH

Repository metadata / workflow change only.

Performance Impact

• No performance impact
• Performance improved
• Performance regression possible

This change only affects GitHub metadata and developer workflow; it does not modify runtime code or execution paths.

Known Issues & Future Work

• Current templates need refinement as new backends and platforms are added.

Test Results

Test Involved Platform

• CPU
• NVIDIA GPU
• MetaX GPU
• Cambricon MLU

Test Involved Backend

• OpenMPI
• MPICH

all_gather.log
all_reduce.log
all_to_all.log
broadcast.log
reduce_scatter.log
send_recv.log

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Checklist

Every contributor must verify every item below before requesting
review. Tick each box only after the check has actually been performed —
do not tick speculatively. If an item truly does not apply, replace the
checkbox with N/A and briefly explain why in an inline comment.

Title, Branch, and Commits

• PR title follows Conventional Commits (e.g. feat: …, fix(nccl): …).
• Branch name follows <type>/xxx-yyyy-zzzz where <type> matches the PR title's Conventional Commits type and words are joined with hyphens (see CONTRIBUTING.md §Branches).
• Each commit message follows Conventional Commits.
• Small PR is a single squashable commit; or, for a large PR, every commit is meaningful, well-formed, and independently reviewable (see CONTRIBUTING.md §Pull Requests).
• No stray merge commits from master — the branch is rebased cleanly on top of the current master.
• No fixup! / squash! / wip commits remain.

Scope and Design

• Changes are minimal — no unrelated modifications were introduced (CONTRIBUTING.md §Code/General).
• No dead code, commented-out blocks, debug prints, printf/std::cout/print(...) left behind, or TODO without an owner and issue link.
• No unrelated formatting churn that would obscure the diff.
• Public API changes (if any) are intentional, documented, and reflected in affected callers/tests.

General Code Hygiene

• The code is self-explanatory; comments were added only where the intent or rationale is non-obvious (CONTRIBUTING.md §Code/General).
• Every modified or added file ends with a single trailing newline (CONTRIBUTING.md §Code/General).
• No trailing whitespace, inconsistent indentation, or mixed formatting styles remain.
• Identifiers referenced in comments or error messages are wrapped in Markdown backticks (e.g. the `AllReduce` implementation) (CONTRIBUTING.md §Code/General).
• All comments and error messages are in English (CONTRIBUTING.md §Code/General).
• Comments and error messages are complete sentences — capitalized first letter, terminal punctuation — unless the language/framework convention says otherwise (CONTRIBUTING.md §Code/General; §Python).

C++ Specific (if C++ files changed)

• N/A- Code follows the Google C++ Style Guide strictly.
• N/A- clang-format (version 16, per .github/workflows/clang-format.yml) has been run against all modified applicable files; the diff is clean.
• N/A- No exceptions are thrown. Error paths use assert with messages that include at least __FILE__, __LINE__, and __func__ (CONTRIBUTING.md §C++).
• N/A- Error and warning message wording follows the LLVM Coding Standards (CONTRIBUTING.md §C++).
• N/A- Constructor initializer list order matches member declaration order (CONTRIBUTING.md §C++).
• N/A- Exactly one blank line between classes, between classes and functions, and between functions (CONTRIBUTING.md §C++).
• N/A- Exactly one blank line between members (functions and variables) within a class (CONTRIBUTING.md §C++).
• N/A- Exactly one blank line before and after the contents of a namespace (CONTRIBUTING.md §C++).

Python Specific (if Python files changed)

• N/A- Code is PEP 8 compliant; ruff check passes cleanly on CI (see .github/workflows/ruff.yml).
• N/A- ruff format --check passes cleanly — if not, run ruff format and commit the result.
• N/A- Comments are complete English sentences, starting with a capital letter and ending with punctuation; Markdown backticks are used for code references (CONTRIBUTING.md §Python).
• N/A- Framework-specific conventions (e.g. lowercase pytest.skip messages without terminal period) are honored where applicable (CONTRIBUTING.md §Python).
• N/A- No blank line between the function signature and the body when there is no docstring or comment (CONTRIBUTING.md §Python).
• N/A- A blank line is present before and after if, for, and similar control-flow statements (CONTRIBUTING.md §Python).
• N/A- A blank line appears before each return, except when it directly follows a control-flow statement (CONTRIBUTING.md §Python).
• N/A- Docstrings (if any) follow PEP 257 (CONTRIBUTING.md §Python).
• N/A- Type hints are added / kept consistent with the surrounding code.

Testing

• All applicable example programs have been built and tested successfully on at least one supported heterogeneous cluster setup.

Build, CI, and Tooling

• N/A- New backends or devices have been added to auto-detection in CMakeLists.txt under if(AUTO_DETECT_DEVICES) or to if(AUTO_DETECT_BACKENDS) if applicable.
• Both CI workflows (clang-format.yml, ruff.yml) are green locally (or expected to be green on CI).

Documentation

• README.md, CONTRIBUTING.md, or inline docs updated when behavior, build flags, or developer workflow changed.
• Any user-visible breaking change is called out explicitly under "Summary" and in the commit/PR title with a ! or BREAKING CHANGE: footer.

Security and Safety

• No secrets, access tokens, internal URLs, customer data, or personal hardware identifiers have been committed.
• Third-party code is license-compatible and attributed.
• No unsafe pointer arithmetic, uninitialized reads, or missing bounds checks were introduced.