text: Title: change return type of `find_batched()` utility function
  Body: Currently, `find_batched()` has the following signature:
  
  ```python
  def find_batched(
      ...
  ) -> List[FindResult]:
  ```
  
  In order to align with `DocumentIndex`, it should be switched to:
  
  ```python
  def find_batched(
      ...
  ) -> FindResultBatched:
  ```
  
  **TODOs:**
  - move the definition of `FindResultBatched` from `index/abstract.py` to `utils/find.py`
  - change the signature as described above
  - fix the imports on all Document Index classes
  - Probably `FindResultBatched` needs a tweak as well: `scores: np.ndarray` probably needs to be changed to allow a union of ndarray, torch.Tensor and tf.Tensor