Fast imports part 3
# What does this PR do?

This is the last PR to make the import of transformers and defer the imports of torch/tensorflow to when is necessary. It does the same work as #9446 but in each itnermediate init, so that
```
from transformers import BertModel
```
only imports torch and not TensorFlow (ans is thus very fast).

The templates are adapted to the new init format, so users adding models don't have to worry about this.
In passing, I noticed that the `tokenization_utils_base` was importing everything at init, so I deferred imports there to only do them when necessary. There might be a few places like this left, but we can address those later on.

Fixes #8733