Raw data preprocessing pipeline step returns exit code 137

[langbart]

The preprocessing GithHub actions' job in the pipeline returns the error "Process completed with exit code 137". It seems that the Docker container running in GithHub actions runs out of memory and the process is killed. A possible solution is to split the preprocessing into two separate jobs (and so in two separate containers) and then merge the final processed data

[langbart]

Closed by PR #87