Move pytrec_eval to its own optional dependencies section (#3470)

Author: yifanmai

setup.cfg

@@ -77,12 +77,14 @@ scenarios =
 metrics =
     google-api-python-client~=2.64  # For perspective_api_client via toxicity_metrics
     numba~=0.56  # For copyright_metrics
-    pytrec_eval==0.5  # For ranking_metrics
     sacrebleu~=2.2.1  # For disinformation_metrics, machine_translation_metrics
     langdetect~=1.0.9  # For ifeval_metrics
     immutabledict~=4.2.0  # For ifeval_metrics
     gradio_client~=1.3  # For bigcodebench_metrics
 
+ranking = 
+    pytrec_eval==0.5  # For ranking_metrics
+
 summarization =
     summ-eval~=0.892  # For summarization_metrics
     bert-score~=0.3  # For summarization_metrics
@@ -338,6 +340,7 @@ all =
     # crfm-helm[seahelm] is excluded because pyonmttok does not support Python 3.12
     # crfm-helm[dev] is excluded because end-users don't need it.
     # crfm-helm[summarize] is excluded because it requires torch<2.0
+    # crfm-helm[ranking] is excluded because pytrec_eval causes build errors (see #3470)
     # TODO(#2280): Add crfm-helm[summarize] back.
 
 # Development only