Fix TF GPT2 `test_onnx_runtime_optimize`
# What does this PR do?

Fix TF GPT2 `test_onnx_runtime_optimize` by skipping 2 test classes.

Current error:

```
tests/models/gpt2/test_modeling_tf_gpt2.py::TFGPT2ModelTest::test_onnx_runtime_optimize
(line 372)  onnxruntime.capi.onnxruntime_pybind11_state.InvalidGraph: [ONNXRuntimeError] : 10 : INVALID_GRAPH : This is an invalid model. In Node, ("tfgpt2_for_sequence_classification_27/GatherV2", GatherV2, "", -1) : ("tfgpt2_for_sequence_classification_27/score/Tensordot:0": tensor(float),"tfgpt2_for_sequence_classification_27/sub:0": tensor(int32),"tfgpt2_for_sequence_classification_27/GatherV2/axis:0": tensor(int32),) -> ("logits": tensor(float),) , Error No Op registered for GatherV2 with domain_version of 10
```
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