code: private int[] convertBatch(int[] batch) {
          int[] conv = new int[batch.length];
          for (int i=0; i<batch.length; i++) {
              conv[i] = sample[batch[i]];
          }
          return conv;
      }
docs: Converts a batch indexing into the sample, to a batch indexing into the
  original function.
  
  @param batch The batch indexing into the sample.
  @return A new batch indexing into the original function, containing only
  the indices from the sample.