[FIX] symbolic shape infer error with onnx-1.11.0

[PeixuanZuo]

When the input shape of the op is uncertain, the inferred output shape is uncertain.
Before onnx1.11.0, the uncertain shape dim is represented by None. In onnx 1.11.0, the uncertain shape is represented by "unk__#index". The method of verifying whether the output shape is certained invalid.
For example, a Where op has three inputs[1,1,min(1024,dim0), min(1024,dim1)],[1,12,dim0,dim1],[], the infered output has shape [1,12,None,None] in onnx1.10 but has shape[1,12,"unk__0",'unk__1'] in onnx1.11.

It's caused by https://github.com/onnx/onnx/blob/main/onnx/shape_inference/implementation.cc#L437, the symbolic shape generated before it's assign to output.

1. Add the logic to distinguish if a symbolic dim represent None.
2. Update the changed onnx model : gpt2_megatron_opt.onnx

Fixes #10761

[jywu-msft]

Can you re-enable tests for symbolic shape infer that were disabled by https://github.com/microsoft/onnxruntime/pull/10441/files#diff-95f6d8002a2b02785895316d5a8268de674a67e7ed3fc0bae0e7274a762a144b
to test this change?
+@liqunfu FYI , can you also help validate?

[thiagocrepaldi]

@baijumeswani That reminds me of the issue you were seeing

[liqunfu]

Thanks for this fix. Please remove this line in the test to enable the test:

onnxruntime/onnxruntime/test/python/onnxruntime_test_python_symbolic_shape_infer.py

Line 30 in a9d9c6b

test_skip_due_to_onnx_1_11_shape_inference_change = ["GPT2", "GPT2_LM_HEAD", "test_GPT2"]

[PeixuanZuo]

Can you re-enable tests for symbolic shape infer that were disabled by https://github.com/microsoft/onnxruntime/pull/10441/files#diff-95f6d8002a2b02785895316d5a8268de674a67e7ed3fc0bae0e7274a762a144b to test this change? +@liqunfu FYI , can you also help validate?

I enable tests which were disabled and pass the unit test.