Sb asma/develop/1361 fix zero div error

[sbAsma]

Description

1. In src/weathergen/datasets/multi_stream_data_sampler.py: Added assertion to ensure workload is properly divisible by the number of workers
2. In src/weathergen/train/trainer.p: Safety fallback to avoid istep being divided by zero.

Issue Number

Closes #1361

Checklist before asking for review

• I have performed a self-review of my code
• My changes comply with basic sanity checks:
  • I have fixed formatting issues with ./scripts/actions.sh lint
  • I have run unit tests with ./scripts/actions.sh unit-test
  • I have documented my code and I have updated the docstrings.
  • I have added unit tests, if relevant
• I have tried my changes with data and code:
  • I have run the integration tests with ./scripts/actions.sh integration-test
  • (bigger changes) I have run a full training and I have written in the comment the run_id(s): launch-slurm.py --time 60
  • (bigger changes and experiments) I have shared a hegdedoc in the github issue with all the configurations and runs for this experiments
• I have informed and aligned with people impacted by my change:
  • for config changes: the MatterMost channels and/or a design doc
  • for changes of dependencies: the MatterMost software development channel

[sbAsma]

@clessig @Jubeku Not sure if I am supposed to tag you or not, doing it just in case ✌️