[doc] add booster docstring and fix autodoc

[ver217]

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1. Some methods of booster misses docstring. Added them.
2. Fix autodoc for class. It will show all methods of a class. No need to refer its function.

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[github-actions]

The code coverage for the changed files is 84%.

Click me to view the complete report

Name                            Stmts   Miss  Cover
---------------------------------------------------
colossalai/booster/booster.py      63     10    84%
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TOTAL                              63     10    84%