Fix iterator over symbol when multiple children have the same name

[ptrendx]

Description

In the current version of MXNet iterating over grouped symbol containing duplicates is impossible:

>>> import mxnet as mx
>>> a = mx.sym.Variable('data')
>>> b = a + a
>>> for c in b.get_children();
>>>    pass
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/runner/.local/lib/python3.6/site-packages/mxnet/symbol/symbol.py", line 93, in <genexpr>
    return (self[i] for i in self.list_outputs())
  File "/home/runner/.local/lib/python3.6/site-packages/mxnet/symbol/symbol.py", line 517, in __getitem__
    raise ValueError('There are multiple outputs with name \"%s\"' % index)
ValueError: There are multiple outputs with name "data"

This PR fixes that.

Checklist

Essentials

Please feel free to remove inapplicable items for your PR.

• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage:
• Unit tests are added for small changes to verify correctness (e.g. adding a new operator)

[piyushghai]

@ptrendx Thanks for fixing this.

@mxnet-label-bot Add [Symbol, backend, pr-awaiting-review]

[Roshrini]

LGTM

[roywei]

@Roshrini @sandeep-krishnamurthy could you help merge this?