chore(backend): format Python codebase with black

[harsh839]

Fixes #444.

Runs black across the backend/ directory to standardise formatting
in line with PEP 8 / the issue description.

What's included

• 18 files reformatted (~492 insertions / ~311 deletions), all under
  backend/.
• The change is purely automated formatting — verified with
  black --check backend/ (24 files left unchanged) and python3 -m py_compile on every touched file. No structural or logic changes.

Notes

• No .black config exists in the repo, so this uses the default
  (88-col, double quotes). Happy to switch the line length/settings if
  the maintainers prefer a project config.
• outputs.py is included since it's part of backend/; a separate fix
  PR (for Outputs.share() fails on Windows: shape/dim wires are sized as int32 but always read as int64 #483/Outputs.share() under-allocates the data wire whenever check_type() promotes the dtype #484) touches the same file — whichever merges first, the
  other may need a trivial rebase.

[harsh839]

@jmplaza could you take a look when you get a chance? This formats the Python backend with black for consistent style.