maintenance: Separate Code into Separate Files

[whoisj]

This change breaks the monolithic src/libtorch.cc into multiple files, with a modern separation of classes into separate header and code files.

No actual code changes were made besides the separation of the code into multiple files.

[whoisj]

pre-commit fails on a file I have not changed 😤

[yinggeh]

pre-commit fails on a file I have not changed 😤

I found that, too. Pre-commit is now using Python 3.14.