[DDP][FSDP2] add unit test to showcase DDP mixed precision with FSDP2 mixed precision

[weifengpy]

🚀 The feature, motivation and pitch

DDP has mixed precision: #92882, code example can be found in torch/testing/_internal/distributed/distributed_test.py _test_ddp_native_mixed_precision

FSDP2 has ignored_param, fully_shard(ignore_params=). We need DDP mixed precision for ignore_params

action: improve test_fully_shard_ignore_params.py to cover the above usecase

Alternatives

No response

Additional context

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cc @H-Huang @awgu @wanchaol @fegin @fduwjj @wz337 @wconstab @d4l3k

[dsashidh]

I'd like to work on this

[weifengpy]

I'd like to work on this

go for it. thanks