[BUGFIX] Fix mrpc dataset idx

[eric-haibin-lin]

Description

Fix the index for the MRPC test set. @pengxin99

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

• 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

[codecov]

Codecov Report

❗ No coverage uploaded for pull request head (fix-mrpc@768ce66). Click here to learn what that means.
The diff coverage is n/a.

[codecov]

Codecov Report

Merging #708 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #708   +/-   ##
=======================================
  Coverage   89.23%   89.23%           
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  Files          65       65           
  Lines        5953     5953           
=======================================
  Hits         5312     5312           
  Misses        641      641

[mli]

Job PR-708/1 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-708/1/index.html