Improve dataset_len error message (#879)

Co-authored-by: Uri Granta <uri.granta@secondmind.ai>

trieste/ask_tell_optimization.py

@@ -435,16 +435,18 @@ def acquisition_state(self) -> StateType | None:
     @classmethod
     def dataset_len(cls, datasets: Mapping[Tag, Dataset]) -> int:
         """Helper method for inferring the global dataset size."""
-        dataset_lens = [
-            tf.shape(dataset.query_points)[0]
+        dataset_lens = {
+            tag: int(tf.shape(dataset.query_points)[0])
             for tag, dataset in datasets.items()
             if not LocalizedTag.from_tag(tag).is_local
-        ]
-        unique_lens, _ = tf.unique(dataset_lens)
+        }
+        unique_lens, _ = tf.unique(list(dataset_lens.values()))
         if len(unique_lens) == 1:
             return int(unique_lens[0])
         else:
-            raise ValueError(f"Expected unique global dataset size, got {unique_lens}")
+            raise ValueError(
+                f"Expected unique global dataset size, got {unique_lens}: {dataset_lens}"
+            )
 
     @classmethod
     def from_record(