Create preprocessed training files: metadata.json is missing ids in the train.txt, test.txt and val.txt

When I run the following -

```
python specter/data_utils/create_training_files.py \
--data-dir data/training \
--metadata data/training/metadata.json \
--outdir data/preprocessed/
```

I get `done getting triplets, success rate:0.00%`

and my data-metrics.json looks like -

```
{
"train": 0,
"val": 0,
"test": 0
}

```
I debugged the code and found that at [line](https://github.com/allenai/specter/blob/541fc2bf8b95118665082a335737508b3ce2c9b8/specter/data_utils/create_training_files.py#L311)
there is a key error when self.metadata is called.
Looks like the ids in train.txt, val.txt and test.txt are not in the metadata.json file

Please help and share the correct metadata.json file