Bug in LASSO AIC BIC formula

Hi, 
I think that the calculation of sigma in this line is wrong:

https://github.com/scikit-learn/scikit-learn/blob/95d4f0841d57e8b5f6b2a570312e9d832e69debc/sklearn/linear_model/_least_angle.py#L1785

According to Zou et al, 2007 Eq. 2.12 the value of sigma is the sigma_ols. It should be calculated as:
```
sigma2 = np.var(R)
```

I hope this is useful.