Avoid shadowing python builtin in tile matmul example

[scal444]

Category

• New feature
• Bugfix
• Breaking change
• Refactoring
• Documentation
• Other (please explain)

Description

In docs, this was rendering a different color than expected because the interpreter picks out sum as a builtin. Seen here

Changelog

Before your PR is "Ready for review"

• [ x] Do you agree to the terms under which contributions are accepted as described in Section 9 the Warp License?
• [x ] Necessary tests have been added
• [ x] Documentation is up-to-date
• [ x] Auto-generated files modified by compiling Warp and building the documentation have been updated (e.g. stubs.py, functions.rst)
• [ x] Code passes ruff check and ruff format --check?

[shi-eric]

Thanks, can you open this MR on the internal repo instead?