Allow gluoncv.auto to work without mxnet

[zhreshold]

Used dummy dynamic modules/class to bypass mxnet import, which also provides hints when specific functions are used without mxnet installed.

Example when import Estimators that requires mxnet:

python -c "from gluoncv import auto; e = auto.estimators.ImageClassificationEstimator({})"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/Users/zhiz/Dev/gluon-cv/gluoncv/auto/estimators/utils.py", line 7, in _dummy_constructor
    raise RuntimeError(self.reason.format(type(self).__name__))
RuntimeError: gluoncv.auto.estimators.ImageClassificationEstimator requires mxnet to be installed which is missing.

[github-actions]

Job PR-1685-ddd813a is done.
Docs are uploaded to http://gluon-vision-staging.s3-website-us-west-2.amazonaws.com/PR-1685/ddd813a/index.html