[python docs] multiple broken cross links in tutorials

[aaronmarkham]

Description

Sphinx builds of the Python API docs have the following warning(s):

/work/mxnet/docs/python_docs/python/build/tutorials/getting-started/crash-course/5-predict.ipynb:72: WARNING: File not found: 'tutorials/getting-started/crash-course/train.md'
/work/mxnet/docs/python_docs/python/build/tutorials/getting-started/crash-course/6-use_gpus.ipynb:214: WARNING: File not found: 'tutorials/getting-started/crash-course/predict.md'
/work/mxnet/docs/python_docs/python/build/tutorials/packages/gluon/loss/custom-loss.ipynb:70: WARNING: File not found: '/api/python/docs/api/gluon/_autogen/mxnet.gluon.loss.L1Loss.html'
/work/mxnet/docs/python_docs/python/build/tutorials/packages/gluon/loss/custom-loss.ipynb:70: WARNING: File not found: '/api/python/docs/api/gluon/_autogen/mxnet.gluon.loss.L2Loss.html'
/work/mxnet/docs/python_docs/python/build/tutorials/packages/gluon/loss/custom-loss.ipynb:71: WARNING: File not found: '/api/python/docs/api/gluon/_autogen/mxnet.gluon.loss.SigmoidBinaryCrossEntropyLoss.html'
/work/mxnet/docs/python_docs/python/build/tutorials/packages/gluon/loss/custom-loss.ipynb:71: WARNING: File not found: '/api/python/docs/api/gluon/_autogen/mxnet.gluon.loss.SoftmaxBinaryCrossEntropyLoss.html'
/work/mxnet/docs/python_docs/python/build/tutorials/packages/gluon/loss/custom-loss.ipynb:72: WARNING: File not found: '/api/python/docs/api/gluon/_autogen/mxnet.gluon.loss.HingeLoss.html'
/work/mxnet/docs/python_docs/python/build/tutorials/packages/gluon/loss/custom-loss.ipynb:269: WARNING: image file not readable: tutorials/packages/gluon/loss/images/inuktitut_1.png
/work/mxnet/docs/python_docs/python/build/tutorials/packages/gluon/loss/custom-loss.ipynb:372: WARNING: image file not readable: tutorials/packages/gluon/loss/images/inuktitut_2.png
/work/mxnet/docs/python_docs/python/build/tutorials/packages/gluon/loss/loss.ipynb:70: WARNING: File not found: '/api/python/docs/api/gluon/_autogen/mxnet.gluon.loss.L1Loss.html'
/work/mxnet/docs/python_docs/python/build/tutorials/packages/gluon/loss/loss.ipynb:70: WARNING: File not found: '/api/python/docs/api/gluon/_autogen/mxnet.gluon.loss.L2Loss.html'
/work/mxnet/docs/python_docs/python/build/tutorials/packages/gluon/loss/loss.ipynb:71: WARNING: File not found: '/api/python/docs/api/gluon/_autogen/mxnet.gluon.loss.SigmoidBinaryCrossEntropyLoss.html'
/work/mxnet/docs/python_docs/python/build/tutorials/packages/gluon/loss/loss.ipynb:71: WARNING: File not found: '/api/python/docs/api/gluon/_autogen/mxnet.gluon.loss.SoftmaxBinaryCrossEntropyLoss.html'
/work/mxnet/docs/python_docs/python/build/tutorials/packages/gluon/loss/loss.ipynb:72: WARNING: File not found: '/api/python/docs/api/gluon/_autogen/mxnet.gluon.loss.HingeLoss.html'

How to reproduce

You can run into the problem following these steps.

1. Run a "lite" binary build.

ci/build.py --docker-registry mxnetci --platform ubuntu_cpu_lite /work/runtime_functions.sh build_ubuntu_cpu_docs

2. Run the Python API docs build:

ci/build.py --docker-registry mxnetci --platform ubuntu_cpu_python /work/runtime_functions.sh build_python_docs

[mxnet-label-bot]

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