Suppress test warnings in transformers tests and fix CUDA CI (#28391)
### Description
Suppress noisy warnings in Python transformers tests and fix missing
test exclusions for CUDA ReduceMax/ReduceMin empty-set tests.
### Motivation and Context
CI logs show ~60+ warnings across transformers tests from PyTorch
TorchScript-based ONNX export deprecation, TracerWarnings, and related
UserWarnings. These are expected when using `dynamo=False` (the legacy
exporter) and obscure actionable test output. Additionally,
`test_reduce_max_empty_set_cuda` and `test_reduce_min_empty_set_cuda`
were not excluded despite being a known CUDA provider limitation (the
CPU provider handles empty sets correctly but CUDA does not).
### Changes
| File | Change |
|---|---|
| `onnxruntime/test/python/transformers/conftest.py` | Add pytest
`filterwarnings` to suppress PyTorch export DeprecationWarning,
TracerWarning, dynamic axes UserWarning, inplace ops UserWarning, and
numpy ndarray UserWarning |
| `onnxruntime/python/tools/transformers/torch_onnx_export_helper.py` |
Wrap `torch.onnx.export` calls in `warnings.catch_warnings()` to
suppress legacy exporter deprecation for non-pytest callers |
| `onnxruntime/test/onnx/gen_test_models.py` | Suppress protobuf
`label()` deprecation warnings from onnx internals |
| `onnxruntime/test/python/transformers/test_parity_t5_mha.py` | Replace
`torch.tensor(ort_output)` with `torch.from_numpy(np.array(ort_output))`
to avoid slow list-of-ndarrays conversion warning |
| `onnxruntime/test/testdata/onnx_backend_test_series_filters.jsonc` |
Add `test_reduce_max_empty_set_cuda` and
`test_reduce_min_empty_set_cuda` to `current_failing_tests` |
|`docs/OperatorKernels.md`|Update operator document |
### Notes
- The CUDA ReduceMax/Min empty-set issue is a pre-existing kernel
limitation where `PrepareForReduce` unconditionally rejects dim=0 axes.
The CPU provider correctly returns identity values (-inf/+inf). A proper
fix requires CUDA kernel changes and is tracked separately.
- All warning suppressions are scoped to known expected warnings from
PyTorch internals, not user code issues.