Update mosaic_fsdp_utils.py

[vchiley]

Porting mosaicml/llm-foundry#1104 to composer.

What does this PR do?

Updates how a new process group is initialized. This version was broken in some version of torch1.XX, but has been working well in torch2+

What issue(s) does this change relate to?

• https://databricks.atlassian.net/browse/GRT-2813
• Porting Update config_moe_args.py llm-foundry#1104 to composer.

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[mvpatel2000]

LGTM! Assuming you tested?

[vchiley]

LGTM! Assuming you tested?

Not yet.
Theres a reason this is a draft 😅

[vchiley]

(I set really short warmups which explains some of the "loss spikes" that come back down)

from here
The newest version is highlighted in aqua
Pink is the true baseline
Light blue is the baseline before mosaicml/llm-foundry#1104

tldr lgtm
although I have no clue what the failing test is...