[CI] Make UT cases in test_comm_ops.py compatible with more devices.

[wwfu109]

At present, the test cases in tests/test_comm_ops.py only support CUDA but don't support other devices. This PR resolve the problem with the following modification:

1. In class vllm.platforms.Platform:
   • add a new class attribute communicate_backend
   • add new virtual functions set_device and device_count
2. In class vllm.platforms.cuda.CudaPlatformBase:
   • specify value to communicate_backend
   • implement functions set_device and device_count
3. In function init_test_distributed_environment in the code tests/utils.py, when calling the function init_distributed_environment, specify value to backend parameter based on the platform, instead of using default value.
4. In code tests/test_comm_ops. py，call device settings and device counting in a more general way.

[github-actions[bot]]

👋 Hi! Thank you for contributing to the vLLM project.

💬 Join our developer Slack at https://slack.vllm.ai to discuss your PR in #pr-reviews, coordinate on features in #feat- channels, or join special interest groups in #sig- channels.

Just a reminder: PRs would not trigger full CI run by default. Instead, it would only run fastcheck CI which starts running only a small and essential subset of CI tests to quickly catch errors. You can run other CI tests on top of those by going to your fastcheck build on Buildkite UI (linked in the PR checks section) and unblock them. If you do not have permission to unblock, ping simon-mo or khluu to add you in our Buildkite org.

Once the PR is approved and ready to go, your PR reviewer(s) can run CI to test the changes comprehensively before merging.

To run CI, PR reviewers can either: Add ready label to the PR or enable auto-merge.

🚀

[mergify[bot]]

This pull request has merge conflicts that must be resolved before it can be merged. Please rebase the PR, @wwfu109.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork