Enable non-ZeRO mode (#7515)
Enabled via `stage=0` which corresponds to DDP.
Remove hardwired path to b16_optimizer.
Enable`torch.autocast` for DDP training
Enable native mixed precision DDP for bfloat16
Update torch.autocast and native mixed precision UTs
<img width="976" height="184" alt="image"
src="https://github.com/user-attachments/assets/92904cdc-e312-46a4-943f-011eb5ab146a"
/>
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Signed-off-by: Olatunji Ruwase <tunji.ruwase@snowflake.com>
Co-authored-by: Stas Bekman <stas00@users.noreply.github.com>
