Large scale pre-trained weights on flow for transfer

[isaacmg]

In order to successfully examine pre-training we need to pre-train a large number of weights on river flow data.

Acceptance Criteria

• [ ] Pre-trained weights for 3 and 4 encoders
• [ ] Pre-trained weights for flow data stashed to GCS trained on at least 50 rivers.
• [ ] Documentation page detailing what rivers model trained on and their evaluation metrics.