[pytest] Avoid flash attn test marker warning

[ArthurZucker]

What does this PR do?

Adds the flash_attn_test marker to the pyproject.toml to remove the following warning:

tests/test_modeling_common.py:3131
  /home/arthur/transformers/tests/test_modeling_common.py:3131: PytestUnknownMarkWarning: Unknown pytest.mark.flash_attn_test - is this a typo?  You can register custom marks to avoid this warning - for details, see https://docs.pytest.org/en/stable/how-to/mark.html
    @mark.flash_attn_test

tests/test_modeling_common.py:3174
  /home/arthur/transformers/tests/test_modeling_common.py:3174: PytestUnknownMarkWarning: Unknown pytest.mark.flash_attn_test - is this a typo?  You can register custom marks to avoid this warning - for details, see https://docs.pytest.org/en/stable/how-to/mark.html
    @mark.flash_attn_test

Allows to de-select them with:

pytest ... -m "not flash_attn_test"

[LysandreJik]

Thanks