RotoGrad

A library for dynamic gradient homogenization for multitask learning in Pytorch

Installation

Installing this library is as simple as running in your terminal

pip install rotograd

The code has been tested in Pytorch 1.7.0, yet it should work on most versions. Feel free to open an issue if that were not the case.

Overview

This is the official Pytorch implementation of RotoGrad, an algorithm to reduce the negative transfer due to gradient conflict with respect to the shared parameters when different tasks of a multi-task learning system fight for the shared resources.

Let's say you have a hard-parameter sharing architecture with a backbone model shared across tasks, and two different tasks you want to solve. These tasks take the output of the backbone z = backbone(x) and fed it to a task-specific model (head1 and head2) to obtain the predictions of their tasks, that is, y1 = head1(z) and y2 = head2(z).

Then you can simply use RotoGrad or RotoGradNorm (RotoGrad + GradNorm) by putting all parts together in a single model.

from rotograd import RotoGradNorm
model = RotoGradNorm(backbone, [head1, head2], size_z, alpha=1.)

where you can recover the backbone and i-th head simply calling model.backbone and model.heads[i]. Even more, you can obtain the end-to-end model for a single task (that is, backbone + head), by typing model[i].

As discussed in the paper, it is advisable to have a smaller learning rate for the parameters of RotoGrad and GradNorm. This is as simple as doing:

optim_model = nn.Adam({'params': m.parameters() for m in [backbone, head1, head2]}, lr=learning_rate_model)
optim_rotograd = nn.Adam({'params': model.parameters()}, lr=learning_rate_rotograd)

Finally, we can train the model on all tasks using a simple step function:

import rotograd

def step(x, y1, y2):
    model.train()
    
    optim_model.zero_grad()
    optim_rotograd.zero_grad()

    with rotograd.cached():  # Speeds-up computations by caching Rotograd's parameters
        pred1, pred_2 = model(x)
        
        loss1 = loss_task1(pred1, y1)
        loss2 = loss_task2(pred2, y2)
        
        model.backward([loss1, loss2])
    
    optim_model.step()
    optim_rotograd.step()
        
    return loss1, loss2

Cooperative mode

In the main paper, a cooperative version of RotoGrad (and RotoGradNorm) is introduced. The intuition is that, after a few epochs where RotoGrad has properly aligned the gradients, it can start focusing on helping to reduce the tasks loss functions as well.

Enabling this mode is as simple as calling model.coop(True/False) after T training epochs. This method works similarly to .train() and .eval() in Pytorch's Modules, setting a boolean variable to tell RotoGrad to enable/disable the cooperative mode.