Optionally use flash-attn's CE loss for metrics

[snarayan21]

What does this PR do?

Resubmission of https://github.com/mosaicml/composer/pull/3394 -- using FA's CE Loss results in lower peak reserved memory usage and higher throughput. We are not adding flash attention as an optional dependency to composer since this makes installs and correct builds messy & take a lot longer.

Manual tests:

Run started on dev (13b-dense-fsdp-fullshard-hsdp-adam-shardedckpt-start-5PtEdK), resumed with this branch (13b-dense-fsdp-fullshard-hsdp-adam-shardedckpt-resume-E5SieL) Run started on this branch (13b-dense-fsdp-fullshard-hsdp-adam-shardedckpt-start-0g8uD4), resumed with dev branch (13b-dense-fsdp-fullshard-hsdp-adam-shardedckpt-resume-TSGoUC) 4th time's the charm :0

Run with torch CE loss (green): tiny-sp-dtms1-32h-wCFWfa Run with FA CE loss (tan): tiny-sp-dtms1-32h-jOfIPL

What issue(s) does this change relate to?

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[snarayan21]

gloo change is in https://github.com/mosaicml/composer/pull/3509