(Further) limit world size in test_fsdp_pure_fp16 (#86280)
Test still fails when run on 5 A100 GPUs, although it works with 5 V100s. Using 4 GPUs seems to be fine.
Followup to #85957
Pull Request resolved: https://github.com/pytorch/pytorch/pull/86280
Approved by: https://github.com/awgu, https://github.com/kit1980
