Optimized EMA implementation (#94820)
This PR proposes an optimized way to do Exponential Moving Average (EMA), which is faster than the current way using `swa_utils.AveragedModel` described in https://pytorch.org/docs/stable/optim.html#custom-averaging-strategies.
This implementation is asynchronous, and is built as an optimizer wrapper so that the EMA weight update happens without any additional CPU/GPU sync, just after optimizer steps, and with limited code changes.
Example usage:
```
model = Model().to(device)
opt = torch.optim.Adam(model.parameters())
opt = EMAOptimizer(opt, device, 0.9999)
for epoch in range(epochs):
training_loop(model, opt)
regular_eval_accuracy = evaluate(model)
with opt.swap_ema_weights():
ema_eval_accuracy = evaluate(model)
```
Here are some benchmarks (time per iteration) on various torchvision models:
|model|this PR iteration time |swa_utils.AveragedModel iteration time| iteration speedup |
|-----|-----------------------------|-----------------------|---------------------------------------------|
| | | | |
|regnet_x_1_6gf|62.73 |67.998 |1.08 |
|regnet_x_3_2gf|101.75 |109.422 |1.08 |
|regnet_x_400mf|25.13 |32.005 |1.27 |
|regnet_x_800mf|33.01 |37.466 |1.13 |
|regnet_x_8gf|128.13 |134.868 |1.05 |
|regnet_y_16gf|252.91 |261.292 |1.03 |
|regnet_y_1_6gf|72.14 |84.22 |1.17 |
|regnet_y_3_2gf|99.99 |109.296 |1.09 |
|regnet_y_400mf|29.53 |36.506 |1.24 |
|regnet_y_800mf|37.82 |43.634 |1.15 |
|regnet_y_8gf|196.63 |203.317 |1.03 |
|resnet101|128.80 |137.434 |1.07 |
|resnet152|182.85 |196.498 |1.07 |
|resnet18|29.06 |29.975 |1.03 |
|resnet34|50.73 |53.443 |1.05 |
|resnet50|76.88 |80.602 |1.05 |
|resnext101_32x8d|277.29 |280.759 |1.01 |
|resnext101_64x4d|269.56 |281.052 |1.04 |
|resnext50_32x4d|100.73 |101.102 |1.00 |
|shufflenet_v2_x0_5|10.56 |15.419 |1.46 |
|shufflenet_v2_x1_0|13.11 |18.525 |1.41 |
|shufflenet_v2_x1_5|18.05 |23.132 |1.28 |
|shufflenet_v2_x2_0|25.04 |30.008 |1.20 |
|squeezenet1_1|14.26 |14.325 |1.00 |
|swin_b|264.52 |274.613 |1.04 |
|swin_s|180.66 |188.914 |1.05 |
|swin_t|108.62 |112.632 |1.04 |
|swin_v2_s|220.29 |231.153 |1.05 |
|swin_v2_t|127.27 |133.586 |1.05 |
|vgg11|95.52 |103.714 |1.09 |
|vgg11_bn|106.49 |120.711 |1.13 |
|vgg13|132.94 |147.063 |1.11 |
|vgg13_bn|149.73 |165.256 |1.10 |
|vgg16|158.19 |172.865 |1.09 |
|vgg16_bn|177.04 |192.888 |1.09 |
|vgg19|184.76 |194.194 |1.05 |
|vgg19_bn|203.30 |213.334 |1.05 |
|vit_b_16|217.31 |219.748 |1.01 |
|vit_b_32|69.47 |75.692 |1.09 |
|vit_l_32|223.20 |258.487 |1.16 |
|wide_resnet101_2|267.38 |279.836 |1.05 |
|wide_resnet50_2|145.06 |154.918 |1.07 |
You can see that in all cases it is faster than using `AveragedModel`. In fact in many cases, adding EMA does not add any overhead since the computation is hidden behind the usual iteration flow.
This is a similar implementation to the one currently in [NVIDIA NeMo](https://github.com/NVIDIA/NeMo).
If the team is interested in merging this, let me know and I'll add some documentation similar to `swa_utils` and tests.
Credits to @szmigacz for the implementation.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/94820
Approved by: https://github.com/janeyx99
