[FEATURE] Add NamedTuple + Dict batchify

[sxjscience]

Description

During the implementation, I find that sometimes we need to have the following functionality:

for sample in data_loader:
     foo(sample.data, sample.label)

Now, the feature is available via NamedTuple, we can combine NamedTuple and the other batchify functions.

Checklist

Essentials

• PR's title starts with a category (e.g. [BUGFIX], [MODEL], [TUTORIAL], [FEATURE], [DOC], etc)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage
• Code is well-documented

Changes

• NamedTuple, tests

cc @dmlc/gluon-nlp-team

[codecov]

Codecov Report

Merging #959 into master will increase coverage by 0.02%.
The diff coverage is 95.12%.

@@            Coverage Diff             @@
##           master     #959      +/-   ##
==========================================
+ Coverage   90.75%   90.78%   +0.02%     
==========================================
  Files          67       67              
  Lines        6384     6423      +39     
==========================================
+ Hits         5794     5831      +37     
- Misses        590      592       +2

Impacted Files	Coverage Δ
src/gluonnlp/data/batchify/batchify.py	96.06% <95.12%> (-0.53%)	⬇️

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