fix: type error while checking context length (#2972)

lm_eval/models/vllm_causallms.py

@@ -486,7 +486,7 @@ class VLLM(TemplateLM):
             inputs = []
             ctxlens = []
             for cache_key, context_enc, continuation_enc in chunk:
-                if full_length := (context_enc + continuation_enc) >= self.max_length:
+                if full_length := len(context_enc + continuation_enc) >= self.max_length:
                     eval_logger.warning(
                         f"Context length {full_length} exceeds max length ({self.max_length}). Truncating context."
                     )