main.py.jinja multi-task and visualization logic

[sgbaird]

Notes from our meeting:

• User selects "multi-task" ==> auto-switch from Default Model type to Custom model type
• When adding existing data, the user needs to specify which task the data corresponds to (column in dataframe)
• For categorical, use https://github.com/facebook/Ax/issues/2752
• Selection of custom model, BOTORCH_MODULAR, model_kwargs, add "," at end of dictionary, comment/uncommenting of lines within model_kwargs
• Assume that multi-task and multi-objective is not possible (or at least not easy for now), https://github.com/facebook/Ax/issues/564#issuecomment-827273606. Maybe have a custom message that mentions the incompatibility or provides a link to this issue ("Not implemented")
• Dummy function for multi-task. (don't use _moo, use _mt instead), inputs: x1, x2, task --> single obj value output ("for simplicity, these have been joined into a single function)
• Synchrony - probably not an issue. As long as get_next_trials works.

Aside: visualization, two different symbols and different line colors