Temp ONNX model files not cleaned up if dispose is not called.

[michaelgsharp]

Right now when we use an ONNX model that is saved inside an ML.NET model, we create a new temp file for the ONNX model. This temp file is only cleaned up if the user manually calls dispose on the loaded ML.NET model. This is a problem because the base ITransformer that is loaded from Model.Load is not disposable. You need to know you have to manually cast it to IDisposable to dispose of it correctly. It also is a problem because the temp file name is not recorded anywhere (other than inside the class itself), so its hard to manually clean it up as well.

[ericstj]

Fixed in #5782