Fix __repr__ for gluon.Parameter

[JulianSlzr]

Description

Printing gluon.Parameter (__repr__) gives an error; this resolves the issue.

Checklist

Essentials

• Passed code style checking (make lint)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage
• [n/a] For user-facing API changes, API doc string has been updated. For new C++ functions in header files, their functionalities and arguments are well-documented.
• To my best knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change

Changes

• Update gluon.Parameter.__repr__

Comments

Printing any gluon.parameters (e.g., print(net.collect_params())) will give:

.../site-packages/mxnet/gluon/parameter.py", line 120, in __repr__
    return s.format(**self.__dict__)
KeyError: 'shape'

instead of

model0_ (
...
  Parameter model0_dense1_weight (shape=(10, 0), dtype=<class 'numpy.float32'>)
  Parameter model0_dense1_bias (shape=(10,), dtype=<class 'numpy.float32'>)
)

This regression in 1.0.0 due to this change, though __repr__ was also at fault for using self.__dict__ directly instead of the properties.

[piiswrong]

Please add an unittest for printing

[JulianSlzr]

Wrote test and rebased, should be good now