Fix initialized weights resetting in Fabric.setup() when using FSDP
#19755
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Codecov Report
Additional details and impacted files@@ Coverage Diff @@
## master #19755 +/- ##
=========================================
- Coverage 84% 59% -26%
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Files 424 419 -5
Lines 34919 34820 -99
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- Hits 29349 20374 -8975
- Misses 5570 14446 +8876 |
carmocca
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Apr 10, 2024
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justusschock
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Apr 11, 2024
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What does this PR do?
Fixes a subtle issue with weight initialization in FSDP triggered by the change #19152. The real cause is that FSDP overrides the
applymethod ofnn.Module:https://github.com/pytorch/pytorch/blob/main/torch/distributed/fsdp/fully_sharded_data_parallel.py/#L567-L605
We call it indirectly here:
pytorch-lightning/src/lightning/fabric/fabric.py
Lines 260 to 262 in 76b691d
and then
root.applyhere:pytorch-lightning/src/lightning/fabric/utilities/device_dtype_mixin.py
Line 122 in 76b691d
which by itself is innocent because all we want to do is apply a function to each submodule.
Since all we want to do is update an attribute recursively, we don't need the parameters to be unshareded anyway. So this PR simply iterates over the modules instead of calling
apply. Consequently, this should also makefabric.setup()faster because no longer does each submodule be unsharded.The added test fails on master.
The bug is critical enough that we will need to prioritize a release after this is merged.
Needs #19756 to unblock failing tests.
📚 Documentation preview 📚: https://pytorch-lightning--19755.org.readthedocs.build/en/19755/
cc @Borda @tchaton @carmocca @justusschock @awaelchli