At Tesla's scale and priorities, they'd probably be less keen on using external cloud providers. Using TPUs at their scale would certainly require Google's AI consultants to supervise which isn't ideal for Tesla.

Not agreeing or disagreeing with their decisions, but if you have the resources, you can certainly design a custom chip that performs a specific type of task very well that beats other competitors. Nvidia's GPUs are have to be reasonably good at training across different NNs. You could have a chip that's exceptional good at training one/two specific types of tasks.

For most companies, this would be a bad idea. However, Tesla knows how to produce hardware.