[Block] Use fused gelu operator

[TaoLv]

Description

Use fused gelu operator.

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 #1082 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #1082   +/-   ##
=======================================
  Coverage   88.31%   88.31%           
=======================================
  Files          67       67           
  Lines        6266     6266           
=======================================
  Hits         5534     5534           
  Misses        732      732

Impacted Files	Coverage Δ
src/gluonnlp/model/block.py	53.19% <100%> (ø)	⬆️

[mli]

Job PR-1082/1 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-1082/1/index.html

[TaoLv]

@eric-haibin-lin I assume that we need work on the master branch of gluonnlp. Please review.

[eric-haibin-lin]

Having nested blocks makes the code hard to read. Shall we directly use F.LeakyRelu?

[mli]

Job PR-1082/2 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-1082/2/index.html

[mli]

Job PR-1082/3 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-1082/3/index.html