[bf16] implement mixed precision mixin and add bf16 support for low level zero

[ver217]

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Closes #3863

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1. Implement mixed precision mixin class described in [FEATURE]: mixed precision mixin #3863 . FP16 and BF16 are supported now.
2. Add bf16 support for low level zero optimizer.
3. Update related 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/amp/naive_amp/mixed_precision_mixin/__init__.py       4      0   100%
colossalai/amp/naive_amp/mixed_precision_mixin/base.py          20      5    75%
colossalai/amp/naive_amp/mixed_precision_mixin/bf16.py          15      1    93%
colossalai/amp/naive_amp/mixed_precision_mixin/fp16.py          49      3    94%
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%
colossalai/zero/low_level/low_level_optim.py                   314     20    94%
tests/test_optimizer/test_adam_kernel.py                        87      1    99%
tests/test_optimizer/test_adam_optim.py                         49      0   100%
tests/test_zero/test_low_level/test_grad_acc.py                 88      1    99%
tests/test_zero/test_low_level/test_zero1_2.py                  90      1    99%
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TOTAL                                                          902     48    95%