ycb_scikit-learn_pr12973

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LassoLarsIC: unintuitive copy_X behaviour
Hi, I would like to report what seems to be a bug in the treatment of the `copy_X` parameter of the `LassoLarsIC` class. Because it's a simple bug, it's much easier to see in the code directly than in the execution, so I am not posting steps to reproduce it.

As you can see here, LassoLarsIC accepts a copy_X parameter.
https://github.com/scikit-learn/scikit-learn/blob/7389dbac82d362f296dc2746f10e43ffa1615660/sklearn/linear_model/least_angle.py#L1487

However, it also takes a copy_X parameter a few lines below, in the definition of ```fit```.
    ```def fit(self, X, y, copy_X=True):```

Now there are two values (potentially contradicting each other) for copy_X and each one is used once. Therefore ```fit``` can have a mixed behaviour. Even worse, this can be completely invisible to the user, since copy_X has a default value of True. Let's assume that I'd like it to be False, and have set it to False in the initialization, `my_lasso = LassoLarsIC(copy_X=False)`. I then call ```my_lasso.fit(X, y)``` and my choice will be silently overwritten. 

Ideally I think that copy_X should be removed as an argument in ```fit```. No other estimator seems to have a duplication in class parameters and fit arguments (I've checked more than ten in the linear models module). However, this would break existing code. Therefore I propose that ```fit``` takes a default value of `None` and only overwrites the existing value if the user has explicitly passed it as an argument to ```fit```. I will submit a PR to that effect.

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