Update Custom C++ and CUDA Operators to use PyTorch stable API/ABI

[mikaylagawarecki]

Description

Migrate advanced_source/cpp_custom_ops.rst to libtorch stable ABI/API. Accompanying change to extension-cpp

Checklist

• The issue that is being fixed is referred in the description (see above "Fixes #ISSUE_NUMBER")
• Only one issue is addressed in this pull request
• Labels from the issue that this PR is fixing are added to this pull request
• No unnecessary issues are included into this pull request.

cc @zou3519

[pytorch-bot]

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[mikaylagawarecki]

remove reference to torch::autograd::Function since we don't support that in stable API/ABI

[zou3519]

I get that you want to push the new stable abi APIs, but I do want the tutorial to read well for both stable abi APIs and the old non-abi-stable APIs. Right now the tutorial is fairly centered around stable abi APIs and reduces the information about the non-abi-stable APIs. Why not have two versions of this tutorial, or, for each code snippet, have one code snippet that is abi-stable and one that is non-abi-stable (this works better if we support collapsible sections, which I'm not sure about)? Then we can have a note at the top to tell the user about the tradeoffs and what they should pick.