Torchscript Deprecation

Torchscript is now fully deprecated in Pytorch 2.9. We should investigate converting usage in MONAI of torch.jit over to torch.export. Not all features and behaviours are supported as it is not the same sort of JIT architecture, and torch.compile is not meant for exporting models to a saved format.

Some MONAI models will have no issues with torch.export.export, however it doesn't capture control flow like Torchscript and is more like tracing. Networks without control flow in their forward definitions can be exported currently, eg.:

import torch
from monai.networks.nets import UNet

net = UNet(spatial_dims=2, in_channels=2, out_channels=1, channels=[4, 8, 16], strides=[2, 2])

t1 = torch.rand(3, 2, 16, 16)
t2 = torch.rand(5, 2, 32, 32)

print(net(t1).shape, net(t2).shape)  # expected shapes

D = torch.export.Dim.DYNAMIC
S = torch.export.Dim.STATIC
enet = torch.export.export(net, args=(t1,), dynamic_shapes=((D, S, D, D),))
torch.export.save(enet, "out.pt2")
enet1 = torch.export.load("out.pt2")

net1 = enet1.module()
print(net1(t1).shape, net1(t2).shape) # same as expected shapes

Work is needed to develop:

• Helper routines to help export then import networks in regular use.
• Helper routines and tests to ensure compatibility.
• Adaptation of existing code to overcome any compatibility issues.
• Tutorials on this new usage and how to design compatible networks.
• Removal of legacy Torchscript usage wherever present, especially when its removal from Pytorch is imminent.

Start with the documented monai.networks.nets.UNet example using torch.export, then audit MONAI's existing torch.jit usage. Identify incompatible networks and behaviors, and develop the listed helper routines, compatibility tests, adaptations, tutorials, and legacy-usage removal. Done means the migration scope is addressed without unsupported Torchscript dependencies.