[refactor] Change names of some variables

[nabenabe0928]

Draft version. This issue is currently low priority and the complete version will be issued as soon as possible.

Before	After	Comment
<something>_hat_optimization and <something>_optimization_pred	``	Not clear what optimization it is
y_hat	y_pred	hat is not informative
n_<something>	num_<something>	Follow Pytorch documents such as here

[nabenabe0928]

https://github.com/automl/Auto-PyTorch/issues/142

[ravinkohli]

We expect the abstract evaluator to be merged to automl_common and it can be handled there. So, do you think we can close this issue? @nabenabe0928