[CI][UT] make device count general

[MengqingCao]

While establishing integration tests for the plugin backend, in order to reuse vLLM's native UTs to the greatest extent, it is necessary to generalize the cuda hard-coded and related functions in vLLM's existing UTs.

This PR is the first in the UT rectification series of PRs.
Mainly changes:

1. Move the cuda_device_count_stateless method to platform so that out-of-tree devices can implement the corresponding method.
2. Provide a method to overwrite the envs.CUDA_VISIBLE_DEVICES environment variable so that vLLM can still obtain the out-of-tree device count through it.

[github-actions]

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