GaussianMixture is runnning too many computation when parameters are alredy provided #26015

In GaussianMixture, if a user is passing some initial values for the weights, means, and precision then there is no need to run the initialization (via kmeans or random) and to estimate the gaussian parameters.

However, currently, we still run these two steps and then discard them.

I think that we can make the estimator more efficient by bypassing these steps in case we already have all the necessary statistics to start the EM algorithm.