[API] add docstrings and initialization to apex amp, naive amp

[flybird11111]

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#3730

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add docstrings and initialization to apex amp, naive amp

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• I have performed a self-review of my code
• I have added thorough tests.
• I have added docstrings for all the functions/methods I implemented

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[github-actions]

The code coverage for the changed files is 78%.

Click me to view the complete report

Name                                               Stmts   Miss  Cover
----------------------------------------------------------------------
colossalai/booster/mixed_precision/fp16_apex.py        5      1    80%
colossalai/booster/mixed_precision/fp16_naive.py       4      1    75%
----------------------------------------------------------------------
TOTAL                                                  9      2    78%

[github-actions]

The code coverage for the changed files is %.

Click me to view the complete report

Name                                               Stmts   Miss  Cover
----------------------------------------------------------------------
colossalai/booster/mixed_precision/fp16_apex.py        6      1    83%
colossalai/booster/mixed_precision/fp16_naive.py       4      1    75%
----------------------------------------------------------------------
TOTAL                                                 10      2    80%