[Numpy] Fix conversion toolkits

[zheyuye]

Description

• Refactor the conversion toolkits to fit the current mxnet and gluonnlp as discussed in Update for Block API #1261 (comment)
• Re-upload the mlm model for bert and generator model for electra

Comments

• TODO: Add test cases for conversion
  cc @sxjscience

[codecov]

Codecov Report

Merging #1274 into numpy will decrease coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##            numpy    #1274      +/-   ##
==========================================
- Coverage   82.49%   82.44%   -0.06%     
==========================================
  Files          38       38              
  Lines        5491     5491              
==========================================
- Hits         4530     4527       -3     
- Misses        961      964       +3     

Impacted Files	Coverage Δ
src/gluonnlp/models/bert.py	84.92% <ø> (+0.50%)	⬆️
src/gluonnlp/models/electra.py	78.94% <ø> (ø)
src/gluonnlp/utils/misc.py	48.18% <0.00%> (-1.45%)	⬇️