[EXAMPLE] Fix MNIST classification example

[r3stl355]

Description

An attempt to run example/gluon/mnist/mnist.py throwing error AttributeError: 'Sequential' object has no attribute 'name_scope'. Removing with net.name_scope(): fixed the problem. Script now runs and completes the training.

Checklist

Essentials

• [X ] PR's title starts with a category (e.g. [BUGFIX], [MODEL], [TUTORIAL], [FEATURE], [DOC], etc)
• [X ] Changes are complete (i.e. I finished coding on this PR)
• [- ] All changes have test coverage - there are no tests but example script runs after this fix
• [X ] Code is well-documented

Changes

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

Comments

• If this change is a backward incompatible change, why must this change be made.
• Interesting edge cases to note here

[mxnet-bot]

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[r3stl355]

Looks like I did something here but can't understand what, any help please? My first PR

[leezu]

Thank you @r3stl355. The name_scope was removed in the master branch for MXNet 2, but the example wasn't updated.