[AMP] Update README to include SQuAD results trained via AMP.

[sxjscience]

Description

Also, all results are available in the google sheets: https://docs.google.com/spreadsheets/d/1BRfZoo1aJr0KhA9FMIxZm2fm07nHaGgCjzukvDnfTJg/edit?usp=sharing

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
• [x] Code is well-documented

cc @dmlc/gluon-nlp-team

[github-actions[bot]]

The documentation website for preview: http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR1429/post_amp_update/index.html

[sxjscience]

Find that the boolean masking triggered an error in MXNet so I used int32 first: https://github.com/apache/incubator-mxnet/issues/19495

[github-actions[bot]]

The documentation website for preview: http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR1429/post_amp_update/index.html

[github-actions[bot]]

The documentation website for preview: http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR1429/post_amp_update/index.html