[WIP] Auto-microbatch fix

[bigning]

What does this PR do?

Fix the auto-microbatch. Before this change, composer added sync_hook to module.register_forward_hook and module.register_full_backward_hook. those hooks are triggered AFTER forward and backward of the original module (not the fsdp wrapper)

issue with previous solution:

let's say the model forward like this:

fsdp_module_0 -> fsdp_module_1 -> fsdp_module_2

if the oom happens on rank 0, right in the middle of fsdp_module_0 and fsdp_module_1. Rank 0 starts this allReduce. Rank 1 will continue run fdsp_module_1, which starts the all_gather. This caused mismatch (rank 0 allReduce vs rank 1 allGather)

fix

So the fix is easy, we just add the hook to pre-foward and pre-backward. So it will do the oom detection before any fsdp allGather, instead of after fsdp allGather.

test

unit test

python -m composer.cli.launcher -n 2 -m pytest -m gpu tests/trainer/test_fsdp.py -k test_fsdp_auto_microbatch

[todo] e2e test