[COREML] Update the json getter

[tqchen]

Description

(Brief description on what this PR is about)

Checklist

Essentials

• Passed code style checking (make lint)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage
• 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

• Feature1, tests, (and when applicable, API doc)
• Feature2, tests, (and when applicable, API doc)

[tqchen]

cc @piiswrong @pracheer

[piiswrong]

I think attr was also referenced in the other files?

[piiswrong]

Also this needs to support both attr and attrs because we need to handle json saved by previous versions

[tqchen]

Already did that( support attr and attrs) they are only refered in layer file

[pracheer]

Looks good to me.

One nitpick suggestion if you think it is useful: can you add some documentation to _get_attrs() with some info on attr/attrs story (like models created w/ which mxnet version will have "attrs" as part of their defn etc)?

[tqchen]

@pracheer updated,

[tqchen]

Merge this ?

[pracheer]

Yes please!