Yahoo Downloader - Why do we need to download stock data day by day?

```
1 Failed download:
['AXP']: YFPricesMissingError('possibly delisted; no price data found (1d 2019-06-01 00:00:00 -> 2019-06-02 00:00:00)')
[*********************100%***********************] 1 of 1 completed

1 Failed download:
['AXP']: YFPricesMissingError('possibly delisted; no price data found (1d 2019-06-02 00:00:00 -> 2019-06-03 00:00:00)')
[*********************100%***********************] 1 of 1 completed
[*********************100%***********************] 1 of 1 completed
[*********************100%***********************] 1 of 1 completed
[*********************100%***********************] 1 of 1 completed
[*********************100%***********************] 1 of 1 completed
```
From the message, we found that YohooFinance processor downloaded the stock data per day, per stock.

The corresponding source code is at [(processor_yahoofinance.py)](https://github.com/AI4Finance-Foundation/FinRL/blob/dcf6bde8d64e1a747673d24dde30dce195bc3cf1/finrl/meta/data_processors/processor_yahoofinance.py#L241). I am not sure why we need to download the stock data day by day rather than all days of interests together, which will accelarate the data downloading more during the training.