Move QDQBert in just PyTorch block
# What does this PR do?

This PR moves `QDQBert` in the main init to make it accessible as soon as PyTorch is installed. This is because otherwise, building the documentation requires to have the quantization modules installed, which is not available in Python 3.9 and makes it harder for a contributor to build the doc. The code of the model file is clean and properly isolates those modules, so this is safe to do. The user will have an explicit error message when trying to use the models, since the base model contains the `requires_backends(self, "pytorch_quantization")` line.
_The documentation is not available anymore as the PR was closed or merged._