[Dy2St][CINN] Dont access cpp flag FLAGS_use_cinn via os.getenv

[SigureMo]

PR Category

Execute Infrastructure

PR Types

Devs

Description

确保所有 FLAGS_use_cinn 的设置与获取都使用单一数据来源，即 C++ 的 flag 状态，这个状态是 Python/C++ 统一的，不应该在 import paddle 之后设置和访问对应的环境变量，这往往会导致数据不同步，难以维护，局部修改注意使用 guard 避免 side effect 意外扩散到后续操作

C++ flag（即在 flags.cc 声明的，FLAGS_ 开头的大多数是），此类主要用于需要 C++ 代码直接感知，应该统一使用以下 API（数据会存在 C++ 侧，不应该存在其他地方）

• 声明 flags.cc
• 设置 paddle.set_flags
• 获取 paddle.get_flags
• guard paddle.base.framework.flag_guard

而非 C++ flag，此类主要用于纯 Python 代码感知，应该使用以下 API（数据会存在环境变量中，不应该存在其他地方）

• 声明 创建 python/paddle/utils/environments.py::EnvironmentVariable 实例 ENV_xxx（如环境变量 AAA_BBB_CCC 应该对应 ENV_AAA_BBB_CCC 实例）
• 设置 ENV_xxx.set
• 获取 ENV_xxx.get
• guard python/paddle/utils/environments.py::EnvironmentVariableGuard

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[Copilot]

Pull Request Overview

This PR ensures that all accesses to the FLAGS_use_cinn flag use a unified C++ flag interface rather than relying on environment variables. Key changes include:

• Adding a flag_guard context manager in framework.py for temporary flag setting.
• Updating in_cinn_mode and related API calls to use paddle.get_flags instead of os.getenv.
• Removing a redundant flag_guard from the tests and standardizing flag naming in test files.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
python/paddle/base/framework.py	Added flag_guard context manager and updated in_cinn_mode to use paddle.get_flags.
python/paddle/jit/dy2static/utils.py	Replaced os.getenv check with a call to in_cinn_mode.
test/ir/pir/test_cse_pass.py	Removed duplicate flag_guard implementation and updated flag naming to CINN_FLAG_NAME.