[bf16] add bf16 support for legacy zero

[ver217]

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1. Add bf16 support for legacy zero
2. Update related tests

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[FrankLeeeee]

LGTM.

[github-actions]

The code coverage for the changed files is 78%.

Click me to view the complete report

Name                                                       Stmts   Miss  Cover
------------------------------------------------------------------------------
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%
tests/test_zero/test_legacy/test_zero_engine.py               74      4    95%
------------------------------------------------------------------------------
TOTAL                                                        792    174    78%