AttributeError: 'Namespace' object has no attribute 'train_datasets'

When I launched the 2nd stage training using :
```
horovodrun -np 4 python pretrain_vcr.py --config config/pretrain-vcr-base-4gpu.json \
--output_dir $PRETRAIN_VCR_EXP
```
An error occured, which is:

```
Traceback (most recent call last):

File "pretrain_vcr.py", line 560, in

main(args)

File "pretrain_vcr.py", line 205, in main

all_dbs = [db for datasets in [opts.train_datasets, opts.val_datasets]

AttributeError: 'Namespace' object has no attribute 'train_datasets'
```
How to fix this problem? Thanks.

Start at pretrain_vcr.py line 205 and compare the parsed arguments with config/pretrain-vcr-base-4gpu.json. Reproduce the command from the issue, trace where train_datasets and val_datasets are expected to be defined, and verify that the second-stage training starts without this AttributeError.

Written by the indexing model from the issue text.