[RFC] Add LayerSkip to AO

[jcaip]

Tracker issue for adding LayerSkip to AO.

This is a training and inference optimization that is similar to layer-wise pruning. It's particularly interesting for LLM inference because it combines very cleanly with speculative decoding to provide up to a 1.86x speedup.

@mostafaelhoushi is interested in adding this to torchtune and is interested in upstreaming a subset of the code to ao. See here for more details. In particular, he's interested in doing this without having to alter the module definition.

This is attractive because this part of LayerSkip is not unique to LLMs and can be used for other models. (@mostafaelhoushi to fill out with relevant results).

What is being proposed:

for LayerSkip there is a training recipe and there is an inference recipe:

• training recipe:
  • layer dropout: This is skipping layers sotchastically during training. this is what I think we can get into torch.ao, because it could benefit all types of models, transformers, CNNs, vision, text, etc. It can speedup training and might improve accuracy.
  • early exit loss: This could also be added to torch.ao and it could help different modalities, but my require more time.
• inference recipe:
  • speculative decoding: yes, this applies to LLM only, has been added to gpt-fast here: pytorch-labs/gpt-fast@main...LayerSkip

[gau-nernst]

Layer dropout during training looks like some form of Stochastic Depth. Some related implementations

• FairSeq: drop a layer entirely for the whole batch. Will enjoy speed up (less computation) https://github.com/facebookresearch/fairseq/blob/920a548ca770fb1a951f7f4289b4d3a0c1bc226f/fairseq/models/wav2vec/wav2vec2.py#L1125-L1146
• Timm: mask samples within a batch independently. Will not enjoy speed up (same computation). There is also scaling https://github.com/huggingface/pytorch-image-models/blob/bf781bc3c208d78a5bb470bbcdd97be8ad0631e2/timm/layers/drop.py#L150-L182

A glance at LayerSkip paper suggests that they mask each sample independently in a batch. Probably need some tricks to see speedups? The torchtune PR implements it by indexing, applying the function, and writing back subset of a batch. Curious to see if the extra overhead is outweighed by less computation during training.

[jcaip]

Yup, the layer dropout aspect of layer skip is basically a version of stochastic depth, that's part of the reason why I'm interested in having it in AO, since a generic stochastic depth function / module would be useful outside of just LLMs.

IIRC when talking to mostafa he is faster when masking + rewriting but the speedups mostly come from the self-speculative decoding part of the technique.

@mostafaelhoushi can you share some benchmarks about the layer dropout implementation specifically when you update the issue? Thanks.

[mostafaelhoushi]

Sorry for the delay from my side.

Other Papers

I would like to mention other papers or models that used layer dropout (aka stochastic depth):

• Vision Models:
  • It was first explored in ResNets by Huang et al, 2016.
  • ConvNext uses it in its training recipe. It uses higher layer dropout rates for larger models 0.1/0.4/0.5/0.5 for ConvNeXt-T/S/B/L respectively trained on ImageNet. However, when training on the larger ImageNet-22K it uses smaller layer dropout rates: 0.0/0.0/0.1/0.1/0.2.
  • Layer dropout is also commonly used in vision transformers. Swin Transformers use higher layer dropout rates for larger models: i.e., 0.2, 0.3, 0.5 for Swin-T, Swin-S, and Swin-B, respectively.
  • Dinov2 also used layer dropout (Cc @danthe3rd) when training.
• NLP Models:
  • LayerDrop increased accuracy of RoBERTa and machine translation Transformer models by applying dropout to every other transformer layer and increased its robustness during inference when removing layers. A dropout rate of 0.2 was used, and it was recommended to use a higher rate, 0.5, for smaller models.
  • Progressive Layer Dropping increased the pretraining speed of BERT by 1.86x by applying a dropout rate that progressively increases every iteration across time and every layer across the model, with a maximum dropout rate of 0.5. The paper found that layer skipping was robust to higher learning rates, which was one of the causes of the training speedup.

Other Implementations

• Regarding per batch vs per sample dropout, we can implement both. However, I would like to mention that
  • when I tried measuring speedup for both I found them to be similar.
  • per batch dropout causes errors when training with FSDP. This is because FSDP likes to find gradients for all modules in a model. Per batch layer dropout will cause a module not to have a forward pass nor a backward pass at an iteration, and this causes FSDP to throw an error. There are mitigations for this, e.g., to overwrite the backward pass (as done here in fariseq2).
• My understanding that the timm implementation doesn't lead to speedup as it replaces samples with zeros rather than skipping compute, right?

Benchmark Results

On TorchTune, I ran this command on a single A100 GPU

$ tune run --nproc_per_node 1 full_finetune_distributed --config llama3/8B_full output_dir=$CKPT_PATH checkpointer.checkpoint_dir=$CKPT_PATH/original checkpointer.output_dir=$CKPT_PATH tokenizer.path=$CKPT_PATH/original/tokenizer.model batch_size=16

and got these measurements:

Maximum Dropout	Dropout Scale Across Layers	Time to Reach 50 Iterations	Speedup
None		01 min 32 sec	1x
0.2	Uniform	01 min 23 sec	1.07x
0.3	Uniform	01 min 17 sec	1.19x
0.5	Uniform	01 min 05 sec	1.42x
0.5	Linear.	TBD	TBD
0.2	Exponential	01 min 30 sec	1.02x
0.5	Exponential	01 min 22 sec	1.12x

I also want to tag @danthe3rd as he guided me to implement the per-sample layer dropout and he has implemented it for Dinov2.

[Xynonners]

https://arxiv.org/abs/2402.17812 DropBP may be an option too (only skip layers in the backward, they claim it leads to better accuracy).