Update torch source markers in pyproject.toml

[JanWerder]

Made the CPU/GPU markers mutually exclusive (extra == 'cpu' and extra != 'gpu' / extra == 'gpu' and extra != 'cpu') to satisfy uv’s “disjoint markers” requirement and let uv sync --extra cpu|gpu pick the right PyTorch index.

Fixes the following error when running uv sync:

error: Failed to parse: `pyproject.toml`
    Caused by: TOML parse error at line 62, column 30
     |
  62 |     { index = "pytorch-cpu", extra = "cpu" },
     |                              ^^^^^
  unknown field `extra`, expected one of `git`, `subdirectory`, `rev`, `tag`, `branch`, `url`, `path`, `editable`, `index`, `workspace`, `marker`