message: update export prediction doc
diff: @@ -872,7 +872,23 @@ Note that the key, slot id, and embedding vector are stored in the sparse model
  ```bash
  hugectr.Model.export_predictions()
  ```
  -This method exports predictions and labels to files for the last several batches of evaluation data.
  +If you want to export the predictions for specified data, using [predict() in inference API](#predict-method) is recommended. This method will export the last batch of evaluation prediction and label to file. If the file already exists, the evaluation result will be appended to the end of the file. This method will only export `eval_batch_size` evaluation result each time. So it should be used in the following way:
  +```
  +for i in range(train_steps):
  + # do train
  + ...
  + # clean prediction / label result file
  + prediction_file_in_current_step = "predictions" + str(i)
  + if os.path.exists(prediction_file_in_current_step):
  + os.remove(prediction_file_in_current_step)
  + label_file_in_current_step = "label" + str(i)
  + if os.path.exists(label_file_in_current_step):
  + os.remove(label_file_in_current_step)
  + # do evaluation and export prediction
  + for _ in range(solver.max_eval_batches):
  + model.eval()
  + model.export_predictions(prediction_file_in_current_step, label_file_in_current_step)
  +```
  **Arguments**
  * `output_prediction_file_name`: String, the file to which the evaluation prediction results will be writen. The order of the prediction results are the same as that of the labels, but may be different with the order of the samples in the dataset. There is NO default value and it should be specified by users.
  