Integration of TorchSharp and LibTorchSharp: exposing LibTorch C++ training API

[artidoro]

This PR fixes #79.

A summary of what this PR does:

1. Merges interesaaat/TorchSharp and interesaaat/LibTorchSharp (preserving as much as possible the file history).
2. Combines native and managed code using the same build system used in ML.NET.
3. Creates a LibTorch.Redist nuget package which redistributes LibTorch C++ frontend of Pytorch as a nuget package (cleared any legal concerns with CELA).

Shortcomings (things that will need fixing):

1. Need to point the official build to an actual nuget feed so that it uploads the packages automatically (this could be a DevOps feed or a MyGet feed or a nuget.org feed).
2. The documentation will need to be fixed. I have just moved it to the docs folder.
3. There are a few tests from interesaaat/TorchSharp that I am currently skipping and which should be re-enabled.
4. The LibTorch.Redist pacakge has only the CPU dlls. It should be straightforward at this point to add the GPU dlls, and create a new pacakge LibTorch.GPU.Redist which contains the additional GPU dlls.

Notice:
We probably need to instruct DevOps to look for new build definitions in the following two yml files (one for the official build and the other for the PR verification build).

Let me know if I should target another branch (other than master) to begin, and of course I am happy to make any changes that you think are needed to check this in.

/cc: @interesaaat @MiguelCaldasMS

[interesaaat]

1. Need to point the official build to an actual nuget feed so that it uploads the packages automatically (this could be a DevOps feed or a MyGet feed or a nuget.org feed).

I believe Miguel already set up this, we probably only need to point to the right build.

[interesaaat]

Can you please update the README with the new info on how to build / platform supported?

[interesaaat]

Also we have Azure Pipelines set up, right? (At least for build and test). Can you please add those too?

[artidoro]

I think Miguel needs to add a new pipeline from DevOps since I have changed the file names of the yml files specifying the build instructions.

[interesaaat]

To me the PR is ok. We will fill a couple of issues to address the known limitations once merged. @migueldeicaza if is ok with I will go ahead and merge it. We will need your help to set up the pipelines (we don't have access to the DevOps account for this repo) and to refresh the docs.

[migueldeicaza]

Agreed - let us merge it now and work on the outstanding issues. Would love to know what happened with the docs that needs work