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

[wwfu109]

At present, the test cases in tests/distributed/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/distributed/test_comm_ops.py，call device settings and device counting in a more general way.

[github-actions]

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[mergify]

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