HybridBlock.export() to return created filenames (#17688)

[athewsey]

Description

Minimum, non-breaking change to alleviate #17688

HybridBlock.export() to return tuple of stored filenames (rather than undefined), to expose this information to user code which would otherwise have to re-implement internal filename selection logic.

Checklist

Essentials

Please feel free to remove inapplicable items for your PR.

• The PR title starts with [MXNET-$JIRA_ID], where $JIRA_ID refers to the relevant JIRA issue created (except PRs with tiny changes) (No JIRA issue, to my knowledge)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage: (Sort of? See Changes section below...)
• Unit tests are added for small changes to verify correctness (e.g. adding a new operator)
• Nightly tests are added for complicated/long-running ones (e.g. changing distributed kvstore)
• Build tests will be added for build configuration changes (e.g. adding a new build option with NCCL)
• Code is well-documented:
• For user-facing API changes, API doc string has been updated.
• For new C++ functions in header files, their functionalities and arguments are documented.
• For new examples, README.md is added to explain the what the example does, the source of the dataset, expected performance on test set and reference to the original paper if applicable
• Check the API doc at https://mxnet-ci-doc.s3-accelerate.dualstack.amazonaws.com/PR-$PR_ID/$BUILD_ID/index.html
• To the best of my knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change

Changes

• Implementations for Python and Perl bindings
• Minor Python docstring update to document change
• Unit test for Python, but not for Perl because existing coverage wasn't there and my Perl is terrible

Comments

• It would probably be preferable to make the output filenames overrideable by user code as well, but this would be a bigger change and add more complexity into the API.
• First-time contributor with a very small change, so sorry if I misunderstood or missed anything!

[leezu]

Please fix

[2020-03-04T09:57:51.222Z] ************* Module mxnet.gluon.block

[2020-03-04T09:57:51.222Z] python/mxnet/gluon/block.py:1120:0: C0303: Trailing whitespace (trailing-whitespace)

[2020-03-04T09:57:51.222Z] 

[leezu]

Friendly ping @athewsey regarding the linting issues

[athewsey]

Whoops, sorry! Should be there now