Gluon base.data_dir() being computed in docs

[zachgk]

In the documentation, we use base.data_dir() to refer to local directories for some functions including the gluon vision cifar100 dataset. However, sphinx processes it to an actual path when generating the function api reference - /home/jenkins_slave/.mxnet/datasets/cifar100. This could confuse users so we should switch to a different way of referencing folders as default parameters.

A quick grep has shown this problem occurs on at least:

• https://mxnet.incubator.apache.org/api/python/gluon/data.html#module-mxnet.gluon.data.vision
• https://mxnet.incubator.apache.org/api/python/gluon/model_zoo.html
• https://mxnet.incubator.apache.org/api/python/contrib/text.html

[mxnet-label-bot]

Hey, this is the MXNet Label Bot. Thank you for submitting the issue! I will try and suggest some labels so that the appropriate MXNet community members can help resolve it. Here are my recommended labels: Gluon, Doc

[zachgk]

@mxnet-label-bot add [Doc, Python]