[Minor] Allow sharing interventions across multiple positions

[frankaging]

Description

We added in keep_last_dim flag into intervention class to represent whether we want to intervene just on the last dimension without any reshaping.

• with this flag set to False, we will concatenate multiple reprs from different positions together and intervene on them as a unified repr.
• with this flag set to True, we intervene on them individually without concatenation. This allows an intervention to be shared across multiple positions easily.

Testing Done

Tutorial has a new test.

Checklist:

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