Training callbacks

Various callbacks to customize training behavior

ShortEpochCallback

 ShortEpochCallback (pct=0.01, short_valid=True)

Fit just pct of an epoch, then stop

learn = synth_learner()
learn.fit(1, cbs=ShortEpochCallback())

epoch	train_loss	valid_loss	time
0	00:00

learn = synth_learner()
learn.fit(1, cbs=ShortEpochCallback(short_valid=False))

epoch	train_loss	valid_loss	time
0	8.432135	00:00

source

GradientAccumulation

 GradientAccumulation (n_acc=32)

Accumulate gradients before updating weights

When the number of steps per accumulation is higher than the number of batches, the parameters (and therefore validation loss) don’t change at all:

learn = synth_learner()
learn.fit(1, lr=0.01, cbs=GradientAccumulation(n_acc=1000))
# ensure valid_loss didn't change
assert learn.recorder.values[-1][1] == learn.recorder.values[0][1]

epoch	train_loss	valid_loss	time
0	20.987558	26.849480	00:00

source

GradientClip

 GradientClip (max_norm:float=1.0, norm_type:float=2.0)

Clip norm of gradients

Normally if we use a learning rate that is too high, our training will diverge. This even happens if we use mixed precision training, which avoid infinities by using dynamic loss scaling, but still diverges:

fp16 = MixedPrecision()

set_seed(99)
learn = synth_learner(lr=1.1, cuda=True)
learn.fit(3, cbs=fp16)

epoch	train_loss	valid_loss	time
0	38.214138	25.269005	00:00
1	377.145508	890.010376	00:00
2	839.392883	9965.747070	00:00

By adding the GradientClip callback, the gradient norm_type (default:2) norm is clipped to at most max_norm (default:1) using nn.utils.clip_grad_norm_, which can avoid loss divergence:

set_seed(99)
learn = synth_learner(lr=1.1, cuda=True)
learn.fit(3, cbs=[GradientClip,fp16])

epoch	train_loss	valid_loss	time
0	2.039428	2.372177	00:00
1	1.402425	0.300728	00:00
2	1.013548	0.332610	00:00

BnFreeze

source

BnFreeze

 BnFreeze (after_create=None, before_fit=None, before_epoch=None,
           before_train=None, before_batch=None, after_pred=None,
           after_loss=None, before_backward=None,
           after_cancel_backward=None, after_backward=None,
           before_step=None, after_cancel_step=None, after_step=None,
           after_cancel_batch=None, after_batch=None,
           after_cancel_train=None, after_train=None,
           before_validate=None, after_cancel_validate=None,
           after_validate=None, after_cancel_epoch=None, after_epoch=None,
           after_cancel_fit=None, after_fit=None)

Basic class handling tweaks of the training loop by changing a Learner in various events

source

set_bn_eval

 set_bn_eval (m:torch.nn.modules.module.Module, use_eval=True)

Set bn layers in eval mode for all recursive children of m.

BnFreeze is useful when you’d like to train two separate models that have a common feature extractor / body. The only part of the model that’s different is the head that you attach for transfer learning.

Learner.freeze() doesn’t suffice here as the BatchNorm layers are trainable by default, and running mean and std of batches are tracked. For feature extractors to fully match, you need to set train_bn=False and these stats need to be frozen as well, which is precisely the function of BnFreeze.

path = untar_data(URLs.MNIST_TINY)
dls  = ImageDataLoaders.from_folder(path, valid_pct=0.2)

https://pytorch.org/tutorials/intermediate/memory_format_tutorial.htmlWe first demonstrate the mismatch of the running stats when using only train_bn=False, by creating a Learner…:

learn1 = vision_learner(deepcopy(dls), resnet18, pretrained=True, train_bn=False)

…and grab the first BatchNorm layer, and store its running mean:

m = learn1.model[0][1].running_mean.clone()

You can see that now that running mean has changed:

learn1.fit(1, lr=0.02)
test_ne(to_detach(learn1.model[0][1].running_mean), m)

epoch	train_loss	valid_loss	time
0	1.148303	0.739404	00:12

When we use the BnFreeze callback, the running statistics will not be changed during training. This is often important for getting good results from transfer learning.

learn1 = vision_learner(deepcopy(dls), resnet18, pretrained=True, train_bn=False, cbs=BnFreeze)
m = learn1.model[0][1].running_mean.detach().clone()
learn1.fit(1, lr=0.02)
test_eq(to_detach(learn1.model[0][1].running_mean), m)

epoch	train_loss	valid_loss	time
0	0.478594	0.270772	00:10