[tests] remove require_non_xpu test markers

[faaany]

What does this PR do?

Both test cases work on XPU now. Below are the rest results:

===================================================================== PASSES ======================================================================
============================================================= short test summary info =============================================================
PASSED tests/test_optimizer.py::OptimizerTester::test_accelerated_optimizer_step_was_skipped
========================================================== PASSES ===========================================================
================================================== short test summary info ==================================================
PASSED tests/test_kwargs_handlers.py::KwargsHandlerTester::test_grad_scaler_kwargs
======================================== 1 passed, 6 deselected, 1 warning in 3.79s =========================================

[faaany]

@muellerzr @yao-matrix

[HuggingFaceDocBuilderDev]

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