[Enhancement] Fix dtype conversion, add sentencepiece support for SQuAD

[eric-haibin-lin]

Description

As title. @davisliang @fierceX please help review.

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

Merging #766 into master will decrease coverage by 0.04%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master     #766      +/-   ##
==========================================
- Coverage   90.77%   90.73%   -0.05%     
==========================================
  Files          64       64              
  Lines        6116     6121       +5     
==========================================
+ Hits         5552     5554       +2     
- Misses        564      567       +3

Impacted Files	Coverage Δ
src/gluonnlp/utils/parameter.py	81.35% <50%> (-3.83%)	⬇️

[mli]

Job PR-766/1 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-766/1/index.html

[mli]

Job PR-766/2 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-766/2/index.html

[mli]

Job PR-766/3 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-766/3/index.html

[mli]

Job PR-766/4 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-766/4/index.html