Fixed broken pyproject.

There was already a set of optional dependencies.

Author: arunchaganty

python-package/pyproject.toml

@@ -38,6 +38,7 @@ documentation = "https://xgboost.readthedocs.io/en/stable/"
 repository = "https://github.com/dmlc/xgboost"
 
 [project.optional-dependencies]
+gpu = ["nvidia-nccl-cu12"]
 pandas = ["pandas"]
 scikit-learn = ["scikit-learn"]
 dask = ["dask", "pandas", "distributed"]