reset accelerate env variables after each test

[pacman100]

What does this PR do?

Context:

For the failing tests in test_trainer_ext, the reason is given below:

1. These tests are run via following command:

python -m pytest -v --make-reports=multi-gpu_tests_torch_cuda_extensions_gpu tests/deepspeed tests/extended

2. Now, as DeepSpeed tests are run first, even though the AcceleratorState is reset during teardown, the env variable set by Accelerate ACCELERATE_USE_DEEPSPEED isn't deleted (if the test isn't a script run as a subprocess) and as such Accelerator object initialization in extended tests create DeepSpeedPlugin leading to them failing with - HFDeepSpeedPlugin raises config mismatch error

3. Simple reproducer:

cd transformers
export CUDA_VISIBLE_DEVICES="0,1"
export RUN_SLOW="yes"
pytest -sv tests/deepspeed/test_deepspeed.py::TrainerIntegrationDeepSpeed::test_hf_ds_config_mismatch tests/extended/test_trainer_ext.py::TestTrainerExt::test_run_seq2seq

This PR fixes it by deleting all the env variables having ACCELERATE in them during test tearDown.

[sgugger]

Thanks!

[muellerzr]

Thanks!

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.