### Description
Expose `OrtValue` class API as first-class citizen.
Make it simular with C++ API.
Enable safe direct native memory access.
Make string tensor manipulation more efficient.
Avoid intermediate structures such as `NamedOnnxValue`,
`DisposableNamedOnnxvalue` and etc.
Provide more examples with `IOBinding`, although `OrtValue` API
potentially makes `IOBinding` redundant for most of scenarios, since
`OrtValue` can be created on top of any memory.
Run all the pre-trained models now with `OrtValue` API as well.
Obsolete `OrtExternalMemory class`. Obsolete IOBinding API that takes
`FixedBufferOnnxValue`.
### Motivation and Context
Make the API efficient and uniform with C++.
This aspires to address:
https://github.com/microsoft/onnxruntime/issues/14918https://github.com/microsoft/onnxruntime/issues/15381
Cc: @Craigacp
### Description
This PR enables building nuget packages locally for on device training
using --build_nuget arg.
This PR also enables the C# bindings by default in the managed package.
If a user triggers any training apis when the native binary is not built
for training, an exception with message "Training is disabled in the
current build. Please build ONNXRuntime from source with the build flags
enable_training and enable_training_on_device. " is thrown.
Build command for creating nuget packes for on device training:
build.bat --enable_training --enable_training_on_device --build_nuget
2 Nuget packages are built
1. Microsoft.ML.OnnxRuntime.Managed
2. Microsoft.ML.OnnxRuntime.Training OR
Microsoft.ML.OnnxRuntime.Training.Gpu
### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
