An error found in influence-models

In influence_models.py line 115, the code is:
`tmp_used = used[:i] + used[i:]`
This line doesn't remove the ith element in used, maybe what you really want is sth like:
`tmp_used = used[:i] + used[i+1:]`
and also need to check whether i+1 is out of range.
For now, because 'tmp_used' is the same as 'used', backward pass actually did nothing.

Start at influence_models.py line 115 and inspect the backward pass around the tmp_used assignment. Verify how the selected index is excluded and how the final index is handled. Done means the backward pass no longer operates on an unchanged used list and remains safe at the list boundary.

Written by the indexing model from the issue text.