pkgutil.extend_path should only be used in files that are shared with other Bob packages

@ssarfjoo pointed out there are these two lines in some __init__.py of this package:

# see https://docs.python.org/3/library/pkgutil.html
from pkgutil import extend_path
__path__ = extend_path(__path__, __name__)

These lines must only be put in __init__.py files which are shared with other Bob packages. For example for this package, it means that only bob/__init__.py and bob/learn/__init__.py must contain these two lines. Because theses files are shared with other Bob packages. It must not be in other __init__.py files so it should be removed from https://gitlab.idiap.ch/bob/bob.learn.pytorch/blob/f1fe9f47c7d42b34e6d660952201ea4288a07f10/bob/learn/pytorch/__init__.py and https://gitlab.idiap.ch/bob/bob.learn.pytorch/blob/f1fe9f47c7d42b34e6d660952201ea4288a07f10/bob/learn/pytorch/extractor/__init__.py because these files only exist in bob.learn.pytorch and not somewhere else.