[MRG] Change the default of precompute from "auto" to False 

The Gram variant has been found to be slower, then the normal update rules. See the discussion in https://github.com/scikit-learn/scikit-learn/pull/3220 . So the default is changed from "auto" to False.
This is the last fit.
1. There might be a case where `self.precompute="auto"`, and we have deprecated it.
2. If `self.precompute=True` and we set `model.precompute=True`, I think we might be going against our principle of believing that computing the Gram, is useless for doing a single fit.

This is basically to make the last fit as fast as possible.