Document generation for operator kernels, enable internal overload of DML EP to initialize on software-only devices (#13428)
### Description
The documentation pipeline does not require an actual GPU, and running
on GPU-capable agents costs more. So to enable running on CPU-only
devices and to potentially consolidate future pipelines, and since the
tests are not actually executed on this device anyway (it just needs to
initialize the EP for the sake of operator kernel enumeration), add an
initialization flag to skip the software device check - this is only an
internal overload not exposed in the public API. See
https://github.com/microsoft/onnxruntime/pull/13308.
### Motivation and Context
- *If it fixes an open issue, please link to the issue here.* NA