[MXNET-949] Module API to Gluon API tutorial

[ThomasDelteil]

Description

Adding a tutorial to show the correspondence between Module and Gluon API to help users transition from the Module API to the Gluon one.

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)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage:
• Nightly tests are added for complicated/long-running ones (e.g. changing distributed kvstore)
• Code is well-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
• To the my best knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change

[kalyc]

Thanks for your contribution @ThomasDelteil
@mxnet-label-bot[pr-awaiting-response]

[sandeep-krishnamurthy]

@ThomasDelteil - Can you please rebase and resolve conflict?

[vandanavk]

@aaronmarkham for review

@sandeep-krishnamurthy Please change the label to pr-awaiting-testing

[aaronmarkham]

I enjoyed this tutorial. This comparison was really needed. There are some minor grammatical/spelling issues and some clarity needs. I feel like each section should have a standard flow:

1. overall intro of what the section does and very high level of how module/gluon differ
2. code examples with intro
3. compare / contrast detail + links to more examples

[Roshrini]

@ThomasDelteil Can you please address review comments and resolve conflicting files?
@sandeep-krishnamurthy Can you please change PR label to "pr-awaiting-response"?

[roywei]

ping @ThomasDelteil , any update on this PR?

[ThomasDelteil]

@roywei going to go through the comments this afternoon and push an update

[anirudhacharya]

should we add a note on how to visualize networks in - symbol graphs vs gluon blocks?

[lupesko]

@ThomasDelteil thanks for the contribution - can you please follow up on the comments?

[ThomasDelteil]

will re-open once I have time to address the comments, thanks all for the reviews 👍

[thomelane]

Much needed tutorial! Great job @ThomasDelteil. Only very minor suggestions.