trunk/88b4e8f8e2c0c3028734c4c7ff9b17fa392d9614: Import TypeVar from typing_extensions (#185708)

Fixes #140914

Imports TypeVar from typing_extensions alongside ParamSpec so both type parameters use the same default-sentinel machinery when another vendored typing_extensions copy has monkeypatched typing internals. This preserves the existing CachedMethod[P, RV] convention while avoiding the import-time TypeError.

Test Plan:

• python3 -m py_compile torch/_inductor/utils.py
• /Users/dejain/.cache/codex-runtimes/codex-primary-runtime/dependencies/python/bin/python3 minimized repro that loads a second typing_extensions copy and defines CachedMethod(Protocol, Generic[P, RV])
• git diff --check

Pull Request resolved: #185708
Approved by: https://github.com/Skylion007, https://github.com/jansel