[FEATURE] Implement mT5 models and conversion scripts; Add T5 documentation

[yongyi-wu]

Description

This PR introduces mT5 models and conversion scripts to GluonNLP codebase. The models, tokenizer, conversion scripts, and test cases reuse most of T5's, so the correctness largely depends on the implementation of T5's components. Besides, docstrings for all T5 building blocks have been added.

Checklist

Essentials

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

Changes

• [x] MT5Model, MT5Inference, and MT5Tokenizer
• [x] Conversion scripts which utilize Huggingface's pretrained models
• [x] Converted models uploaded to S3
• [x] T5 documentation

cc @dmlc/gluon-nlp-team

[codecov[bot]]

Codecov Report

Merging #1506 (eddda7d) into master (f44ec96) will decrease coverage by 0.12%. The diff coverage is 95.68%.

@@            Coverage Diff             @@
##           master    #1506      +/-   ##
==========================================
- Coverage   86.35%   86.23%   -0.13%     
==========================================
  Files          54       55       +1     
  Lines        7388     7502     +114     
==========================================
+ Hits         6380     6469      +89     
- Misses       1008     1033      +25     

Impacted Files	Coverage Δ
src/gluonnlp/attention_cell.py	88.39% <ø> (ø)
src/gluonnlp/models/mt5.py	95.49% <95.49%> (ø)
src/gluonnlp/models/__init__.py	100.00% <100.00%> (ø)
src/gluonnlp/models/t5.py	93.67% <100.00%> (+0.01%)	:arrow_up:
src/gluonnlp/models/transformer.py	98.52% <100.00%> (+<0.01%)	:arrow_up:
src/gluonnlp/data/loading.py	75.84% <0.00%> (-7.55%)	:arrow_down:

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[github-actions[bot]]

The documentation website for preview: http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR1506/ec54d05bd8331c5ee50b0ff169818952dd79846c/index.html

[github-actions[bot]]

The documentation website for preview: http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR1506/eddda7db3c822ff773658c613df8ad1128b41d87/index.html