feat(python): support list of list fields from pydantic schema (#747)

For object detection, each row may correspond to an image and each image
can have multiple bounding boxes of x-y coordinates. This means that a
`bbox` field is potentially "list of list of float". This adds support
in our pydantic-pyarrow conversion for nested lists.

python/lancedb/pydantic.py

@@ -164,6 +164,9 @@ def _py_type_to_arrow_type(py_type: Type[Any]) -> pa.DataType:
         return pa.date32()
     elif py_type == datetime:
         return pa.timestamp("us")
+    elif py_type.__origin__ in (list, tuple):
+        child = py_type.__args__[0]
+        return pa.list_(_py_type_to_arrow_type(child))
     raise TypeError(
         f"Converting Pydantic type to Arrow Type: unsupported type {py_type}"
     )