Add PyTorch .sgn() tensor operation documentation

[ylevental]

Description

Added term entry for PyTorch's .sgn() function at content/pytorch/concepts/tensor-operations/terms/sgn/sgn.md.

The function computes element-wise sign:

• Real tensors: returns -1, 0, or 1
• Complex tensors: returns unit vector (x/|x|)

Includes three examples demonstrating 1D real tensors, 2D tensors, and complex number behavior.

Metadata follows patterns from similar operations (.neg(), .positive()): Computer Science and Data Science subjects, Deep Learning/Methods/PyTorch/Tensor tags.

Issue Solved

Closes #8082

Type of Change

• Adding a new entry

Checklist

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