Add submodules to sys.path to fix autodoc's "No module named 'health_azure'"

[JacopoTeneggi]

Hi @fepegar ! I ran make html from your new branch and noticed I was still getting the following warning:

WARNING: autodoc: failed to import class 'config.SegmentationModelBase' from module '[InnerEye.ML](https://innereye.ml/)'; the following exception was raised:
Traceback (most recent call last):
  File "/cis/home/jteneggi/anaconda3/envs/InnerEye/lib/python3.7/site-packages/sphinx/util/inspect.py", line 324, in safe_getattr
    return getattr(obj, name, *defargs)
AttributeError: module '[InnerEye.ML](https://innereye.ml/)' has no attribute 'config'
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
  File "/cis/home/jteneggi/anaconda3/envs/InnerEye/lib/python3.7/site-packages/sphinx/ext/autodoc/importer.py", line 71, in import_object
    obj = attrgetter(obj, attrname)
  File "/cis/home/jteneggi/anaconda3/envs/InnerEye/lib/python3.7/site-packages/sphinx/ext/autodoc/__init__.py", line 246, in get_attr
    return autodoc_attrgetter(self.env.app, obj, name, *defargs)
  File "/cis/home/jteneggi/anaconda3/envs/InnerEye/lib/python3.7/site-packages/sphinx/ext/autodoc/__init__.py", line 2078, in autodoc_attrgetter
    return safe_getattr(obj, name, *defargs)
  File "/cis/home/jteneggi/anaconda3/envs/InnerEye/lib/python3.7/site-packages/sphinx/util/inspect.py", line 340, in safe_getattr
    raise AttributeError(name)
AttributeError: config
WARNING: autodoc: failed to import module 'config' from module '[InnerEye.ML](https://innereye.ml/)'; the following exception was raised:
No module named 'health_azure'

I propose to call fixed_paths.add_submodules_to_path() in conf.py as in ML/runner.py to make sure all submodules are imported correctly.

[fepegar]

Oops I didn't realise merging #699 would close this PR. I'm really sorry about that! Would it be easy to re-create this branch from main and submit again?

[JacopoTeneggi]

Done! It is here #704