The first tpu they made was inference only. Everything since has been used for training. I think that means they weren't using it for training in 2015 but rather 2017 based on Wikipedia.

Some details here: https://news.ycombinator.com/item?id=42392310

no. for internal training most work is done on TPUs, which have been explicitly designed for high performance training.

I've heard its a mixture because they can't source enough in-house compute