Add PyTorch .sign() tensor operation term entry

[ylevental]

Description

Added term entry for PyTorch's .sign() method under content/pytorch/concepts/tensor-operations/terms/sign/sign.md.

The entry documents the method that returns the sign of each tensor element (-1, 0, or 1), covering:

• Method signature with input and optional out parameters
• Practical example demonstrating behavior with positive, negative, and zero values
• Expected output showing sign computation

Follows existing PyTorch term entry patterns (.abs(), .ceil()).

Issue Solved

Closes #8081

Type of Change

• Adding a new entry

Checklist

• [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.