split issue templates (#16558)

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Create a report to help us improve
+title: ''
+labels: 'Bug'
+assignees: ''
+
+---
+## Description
+(A clear and concise description of what the bug is.)
+
+### Error Message
+(Paste the complete error message, including stack trace.)
+
+## To Reproduce
+(If you developed your own code, please provide a short script that reproduces the error. For existing examples, please provide link.)
+
+### Steps to reproduce
+(Paste the commands you ran that produced the error.)
+
+1.
+2.
+
+## What have you tried to solve it?
+
+1.
+2.
+
+## Environment
+
+We recommend using our script for collecting the diagnositc information. Run the following command and paste the outputs below:
+```
+curl --retry 10 -s https://raw.githubusercontent.com/dmlc/gluon-nlp/master/tools/diagnose.py | python
+
+# paste outputs here
+```

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Suggest an idea for this project
+title: ''
+labels: 'Feature request'
+assignees: ''
+
+---
+
+## Description
+(A clear and concise description of what the feature is.)
+- If the proposal is about a new model, provide description of what the model is.
+- If the proposal is about an API, provide mock examples if possible.
+
+## References
+- list reference and related literature
+- list known implementations

.github/ISSUE_TEMPLATE/flaky_test.md

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+---
+name: Flaky test
+about: Report a flaky test
+title: ''
+labels: 'Flaky'
+assignees: ''
+
+---
+## Description
+(The location and name of the flaky test.)
+
+## Occurrences
+(Links to the known occurrences.)
+
+## What have you tried to solve it?
+
+1.
+2.