[Core][Env Cache] Enable environment variable cache for Worker as well

[Jialin]

Purpose

Enable environment variable caching in Worker at the end of model_load.

This should be helpful for use cases who use ray of torchrun to spawn processes.

We also avoid duplicated functools.cache wrappings and enhanced unit tests in this PR.

Context
We firstly introduced and enabled environment variable caching in #26146 in EngineCore and Executor, this PR is a followup PR suggested by @youkaichao (#26146 (review))

Test Plan / Test Result

CI

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[gemini-code-assist]

Code Review

This pull request enables environment variable caching within the Worker process by calling enable_envs_cache() at the end of the init_device method. This change aligns with a performance optimization already implemented in other components like EngineCore and Executor. The implementation is straightforward, correctly placed in the worker's initialization lifecycle, and should effectively reduce overhead from environment variable lookups in worker processes. The change appears correct and safe.

[chatgpt-codex-connector]

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[22quinn]

👍

[Jialin]

#27896 should be a better replacement of this PR, so close it for now.