Fix regex pattern for includenodoc to parse raw docstring

[sonnyding1]

This PR solves the problem in pytorch/tutorials#3619. In the "Learning PyTorch with Examples" tutorial, the autograd code block started with rimport, where it should be import. This happened because the custom includenodoc directive didn't include the r as part of a raw docstring like

r"""
PyTorch: Tensors and autograd
...
"""

[svekars]

Thank you! Did you test this?

[sonnyding1]

Yes, I replaced the pytorch_sphinx_theme2 package in tutorials with my current branch, and it's parsing correctly now.