Fix TypeError: Couldn't cast array of type...

To fix the issue I set the struct type to be string types instead (they can be loaded using json.loads() in python if needed). This concerns those fields: `doc,` `metadata`, and `attributes`.

Alternatively we could define a new type `json` for `datasets` (feel free to open a PR !), let me know what you think

README.md

@@ -7,6 +7,22 @@ size_categories:
 - 1B<n<10B
 license: odc-by
 pretty_name: OLMoE Mix (September 2024)
+dataset_info:
+  features:
+  - name: id
+    dtype: string
+  - name: text
+    dtype: string
+  - name: added
+    dtype: string
+  - name: attributes
+    dtype: string
+  - name: created
+    dtype: string
+  - name: doc
+    dtype: string
+  - name: metadata
+    dtype: string
 ---
 
 # OLMoE Mix (September 2024)