pydra task input types cause workflow connection problem

[yibeichan]

when creating the 7th tutorial (two-level glm) for pydra, see this PR and this issue, we found that pydra is strict with its task input.

if the task input is a list of unidentifiable types for pydra such as a list of pandas.DataFrame or FirstLevelModel, will cause connection problems in the workflow that the output can't be collected or passed to the next node.

@satra mentions

in pydra-ml we pass other non-standard objects (for example scikit-learn pipelines: https://github.com/nipype/pydra-ml/blob/master/pydra_ml/tasks.py#L103 - hence there must be something specific that's at work here)

@djarecka finds that

adding an extra elif block for input that is a data frame helped, at least in my simple example that I was using for testing. Will check for the entire workflow later. some issues raise only when running with jupyter notebook not .py script