[bf16] cpu adam and hybrid adam optimizers support bf16

[ver217]

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#3839

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1. CPU adam and hybrid adam support native bf16.
2. Refactor adam kernel and optimizer tests.

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[github-actions]

The code coverage for the changed files is 95%.

Click me to view the complete report

Name                                       Stmts   Miss  Cover
--------------------------------------------------------------
colossalai/nn/optimizer/cpu_adam.py           68      3    96%
colossalai/nn/optimizer/fused_adam.py         55     10    82%
colossalai/nn/optimizer/hybrid_adam.py        63      3    95%
tests/test_optimizer/test_adam_kernel.py      87      1    99%
tests/test_optimizer/test_adam_optim.py       49      0   100%
--------------------------------------------------------------
TOTAL                                        322     17    95%