add support for dict outputs (select the first value)

[leogagnon]

Description

Add simple (and naive) for dictionary output of modules. Similar to how tupless were dealt with, I just select the first value of the dict. Only made changes to BaseModel. I'm not yet super familiar with the code so there might be some other modification needed.

Testing Done

It works with my model which has module dict outputs (for ZeroIntervention and LoreftIntervention). Didn't do extensive testing as this felt quite straightforward.

Checklist:

• [ ] My PR title strictly follows the format: [Your Priority] Your Title
• [ ] I have attached the testing log above
• [ ] I provide enough comments to my code
• [ ] I have changed documentations
• [ ] I have added tests for my changes

[aryamanarora]

LGTM, pinging @frankaging: have you ever used modules which output dicts? Hasn't come up for me but this fix seems reasonable.

[frankaging]

LGTM! Later, we might want to further remove the assumption of intervening on the first element by creating field in the config to provide the intervening keyword.

[frankaging]

@leogagnon Thanks!