757: add return_ci=True in sensitivity

[lee-junseok]

Proposed changes

Issue 757

return_ci=True argument is not passed in the get_ate_ci method of Sensitivity class object and running sensitivity_analysis gives a ValueError for a Tlearner object. Adding return_ci=True to the get_ate_ci method with another nested try/except layer since not all estimate_ate methods of learners that is to be run in get_ate_ci method of Sensitivity take the return_ci argument.

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