* fix: guard torch.AcceleratorError for compatibility with torch < 2.8.0
torch.AcceleratorError was introduced in PyTorch 2.8.0. Accessing it
directly raises AttributeError on older versions. Use a try/except
fallback at module load time, consistent with the existing pattern used
for OOM_EXCEPTION.
* fix: address review feedback for AcceleratorError compat
- Fall back to RuntimeError instead of type(None) for ACCELERATOR_ERROR,
consistent with OOM_EXCEPTION fallback pattern and valid for except clauses
- Add "out of memory" message introspection for RuntimeError fallback case
- Use RuntimeError directly in discard_cuda_async_error except clause
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