Refine ONNX model path in Nuget CI

[wschin]

Fix #12040. This PR removes most of the opset uses in C# test code. The only use of opset now is to workaround a special filename only used in a opset9 test.

if (onnxModelFileName.Contains("opset9") && onnxModelFileName.Contains("LSTM_Seq_lens_unpacked"))
  {
    Console.WriteLine($"Change data folder's pattern for LSTM_Seq_lens_unpacked in opset 9.");
    testDataDirNamePattern = "seq_lens*"; // discrepancy in data directory
}

After this PR, we can expose opset from InferenceSesson and completely remove opset in C# test code; for example,

if (session.GetOpSet("onnx") == 9 && onnxModelFileName.Contains("LSTM_Seq_lens_unpacked"))
  {
    Console.WriteLine($"Change data folder's pattern for LSTM_Seq_lens_unpacked in opset 9.");
    testDataDirNamePattern = "seq_lens*"; // discrepancy in data directory
}

It also adds a new environment variable to define model path to test Nuget packages, so it's more explicit to see where the test models come from.

[garymm]

It seems the missing opset from the file path is causing problems: https://aiinfra.visualstudio.com/Lotus/_build/results?buildId=216306&view=logs&j=91a8ea80-f671-5bc1-2729-69581be48524&t=6dd1ca96-d683-54be-88cc-0ee3472572fe

[wschin]

My intention was to remove "opset" in whole test code and pass in model folder path as environment variable. The "opset" is only used to display error message...

[wschin]

Sure. For this comment, I will do

                    if (onnxModelFileName.Contains("opset9") && onnxModelFileName.Contains("LSTM_Seq_lens_unpacked");
                    {
                        Console.WriteLine($"Change data folder's pattern for LSTM_Seq_lens_unpacked in opset 9.");
                        testDataDirNamePattern = "seq_lens*"; // discrepancy in data directory
                    }

[edgchen1]

Sure. For this comment, I will do

                    if (onnxModelFileName.Contains("opset9") && onnxModelFileName.Contains("LSTM_Seq_lens_unpacked");
                    {
                        Console.WriteLine($"Change data folder's pattern for LSTM_Seq_lens_unpacked in opset 9.");
                        testDataDirNamePattern = "seq_lens*"; // discrepancy in data directory
                    }

or maybe get the two ancestor directories (e.g. with Path.GetDirectoryName) to match more exactly