[Bugfix]: During testing, use pytest monkeypatch for safely overriding the env var that indicates the vLLM backend

[afeldman-nm]

Issue #5208 details how tests/kernels/test_attention_selector.py attempts to set VLLM_ATTENTION_BACKEND in order to override automatic backend selection during a unit test, and then reset VLLM_ATTENTION_BACKEND to its prior value as part of test cleanup; however, an unhandled edge-case (VLLM_ATTENTION_BACKEND unset prior to test) causes the test to leave VLLM_ATTENTION_BACKEND set in the environment, which in turn breaks other unrelated tests that are incompatible with the backend override specified by VLLM_ATTENTION_BACKEND.

To fix this issue & avoid it in the future, this PR

• Introduces a context manager env_var_fixture(var_name: str, value: str) -> Iterator[None] in tests/utils.py which temporarily sets an environment variable, and then resets it upon exiting the context manager. The variable will be unset upon exiting the context manager, if it was unset when the context manager was entered.

This PR is also helps address #187 , insofar as that the encoder/decoder unit tests in PR #4888 were failing due to #5208

Usage:

with env_var_fixture("my_var","my_val"):
    # code that depends on my_val == "my_val"

# my_var is returned to original value or unset

• Introduces tests/kernels/utils.py for kernels-related helper functions
• In tests/kernels/utils.py, introduces a context manager backend_override_fixture(backend_name: str) -> Iterator[None] which wraps env_var_fixture() in order to create a test fixture for safely forcing backend overrides by setting VLLM_ATTENTION_BACKEND. Upon exiting the context manager, the backend override will be removed. Usage:

with backend_override_fixture("backend_name"):
    # code that depends on vLLM backend

# VLLM_ATTENTION_BACKEND is returned to original value
# or unset

• Updates tests/kernels/test_attention_selector.py to use backend_override_fixture() in lieu of all instances where VLLM_ATTENTION_BACKEND was being explicitly set and reset.

FIX #5208

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[sroy745]

2 minor comments. Otherwise LGTM.

[DarkLight1337]

Thanks for the fix!