Refactor providers into separate libraries

[RyanUnderhill]

This removes most of the #if USE_CUDA and #if USE_DML blocks for the model handling code. Device memory management is also handled through the DeviceSpan structure and now all data copying is done in a device independent manner.

It's a huge change, and there will be some rough edges when submitted. Goal is to unblock other people needing the changes and then to make larger improvements in future prs.

[natke]

Can you enumerate the places where any #ifdefs remain and why they need to be there please

And what impact will the rough edges have and can they be smoothed before you merge this PR?

[baijumeswani]

First pass through the code

[RyanUnderhill]

Can you enumerate the places where any #ifdefs remain and why they need to be there please

And what impact will the rough edges have and can they be smoothed before you merge this PR?

There are some #if USE_CUDA in our tests, this shouldn't be a problem
There are two #if USE_DML, one in generators.cpp due to it not being a shared library and a second in model.cpp for a similar reason. Making it into a shared library should factor those out and remove the #ifs (the shared library's existence takes the place of the #if, since when it's static you will fail to build without the #if)

The rough edges are just expected simple bugs we'll find and easily fix that I can't find in advance.