[bf16] add bf16 support

[ver217]

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1. Add bf16 support for fused adam kernel
2. Add bf16 support for base adam optimizers, include FusedAdam, CPUAdam and HybridAdam
3. Implement a helper class MixedPrecisionMixin, by which we can implement mixed precision optimizers easily
4. Add bf16 support for low level zero, gemini and legacy zero
5. Update related booster plugins

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The code coverage for the changed files is 87%.

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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/booster/plugin/gemini_plugin.py                     109      9    92%
colossalai/booster/plugin/low_level_zero_plugin.py              94      8    91%
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/gemini/gemini_ddp.py                           402     67    83%
colossalai/zero/gemini/gemini_optimizer.py                     195     32    84%
colossalai/zero/legacy/init_ctx/init_context.py                147      4    97%
colossalai/zero/legacy/sharded_model/_utils.py                  57     24    58%
colossalai/zero/legacy/sharded_model/sharded_model_v2.py       299    108    64%
colossalai/zero/legacy/sharded_optim/sharded_optim_v2.py       215     34    84%
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_gemini/test_optim.py                      130      1    99%
tests/test_zero/test_legacy/test_zero_engine.py                 74      4    95%
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%
--------------------------------------------------------------------------------
TOTAL                                                         2624    339    87%