[Term Entry] PyTorch Tensor Operations: .mm()

Created a new term entry for the .mm() method under Tensor Operations in PyTorch.

Closes #5059

[x] All writings are my own.
[x] My entry follows the Codecademy Docs style guide.
[x] My changes generate no new warnings.
[x] I have performed a self-review of my own writing and code.
[x] I have checked my entry and corrected any misspellings.
[x] I have made corresponding changes to the documentation if needed.
[x] I have confirmed my changes are not being pushed from my forked main branch.
[x] I have confirmed that I'm pushing from a new branch named after the changes I'm making.
[x] I have linked any issues that are relevant to this PR in the Issues Solved section.

Codecademy / docs