nbdev_build_docs fails on colab specific vendor tags inside jupyter notebook cells

[butchland]

nbdev_build_docs fails on notebooks run on colab due to colab adding some vendor specific entry
application/vnd.google.colaboratory.intrinsic+json which for some reason is added only on the learner.summary cells.

(fastai2) xxx:fastai_xla_extensions zzzz$ nbdev_clean_nbs
(fastai2) xxx:fastai_xla_extensions zzzz$ nbdev_build_lib
Converted 00_core.ipynb.
Converted index.ipynb.
(fastai2) xxx:fastai_xla_extensions zzzz$ nbdev_build_docs
converting: /yyyy/fastai_xla_extensions/nbs/00_core.ipynb
converting: /yyyy/fastai_xla_extensions/nbs/index.ipynb
{'type': 'string'} is not valid under any of the given schemas

Failed validating 'oneOf' in execute_result['properties']['data']['patternProperties']['^(?!application/json$)[a-zA-Z0-9]+/[a-zA-Z0-9\\-\\+\\.]+$']:

On instance['cells'][27]['outputs'][0]['data']['application/vnd.google.colaboratory.intrinsic+json']:
{'type': 'string'}
{'type': 'string'} is not valid under any of the given schemas

Failed validating 'oneOf' in execute_result['properties']['data']['patternProperties']['^(?!application/json$)[a-zA-Z0-9]+/[a-zA-Z0-9\\-\\+\\.]+$']:

On instance['cells'][40]['outputs'][0]['data']['application/vnd.google.colaboratory.intrinsic+json']:
{'type': 'string'}
Traceback (most recent call last):
  File "/yyyy/anaconda3/envs/fastai2/bin/nbdev_build_docs", line 8, in <module>
    sys.exit(nbdev_build_docs())
  File "/yyyy/anaconda3/envs/fastai2/lib/python3.7/site-packages/fastscript/core.py", line 76, in _f
    func(**args.__dict__)
  File "/yyyy/anaconda3/envs/fastai2/lib/python3.7/site-packages/nbdev/cli.py", line 223, in nbdev_build_docs
    notebook2html(fname=fname, force_all=force_all, n_workers=n_workers)
  File "/yyyy/anaconda3/envs/fastai2/lib/python3.7/site-packages/nbdev/export2html.py", line 580, in notebook2html
    raise Exception(msg + '\n'.join([f.name for p,f in zip(passed,files) if not p]))
Exception: Conversion failed on the following:
00_core.ipynb
index.ipynb
(fastai2) xxx:fastai_xla_extensions zzzz$ 

Currently the workaround is run nbdev_clean_nbs --clear_all CLEAR_ALL but this also removes the outputs which might not be what you want especially for documentation purposes.

[github-actions]

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[butchland]

Adding a sample notebook that triggers this error.

nbdev version is nbdev==0.2.28