[Bug] torch.cat fails for linear operators

[Christopher Yeh]

🐛 Bug

torch.cat fails for linear operators.

To reproduce

** Code snippet to reproduce **

from linear_operator.operators import DiagLinearOperator
import torch

D = DiagLinearOperator(torch.randn(2, 3, 100))  # Represents an operator of size 2 x 3 x 100
torch.cat([D, D], dim=-2)

** Stack trace/error message **

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/ubuntu/miniconda3/envs/env/lib/python3.12/site-packages/linear_operator/operators/_linear_operator.py", line 2948, in __torch_function__
    raise NotImplementedError(f"torch.{name}({arg_classes}, {kwarg_classes}) is not implemented.")
NotImplementedError: torch.cat(list, dim=int) is not implemented.

Expected Behavior

According to the documentation, torch.cat should work on linear operators.

System information

Please complete the following information:

• LinearOperator version: 0.5.2
• PyTorch Version: 2.2.1
• OS: Ubuntu 20.04.6 LTS

[Max Balandat]

Hmm interesting. Yeah not sure why the docs contain this, seems like this was never implemented. Support is there for many unary or binary operators, but torch.cat operates on a list of objects rather than a LinearOperator directly. @gpleiss have you considered this and similar operators in the past?

[Christopher Yeh]

There is a CatLinearOperator that implements what is needed. I wish that linear_operator could implement LinearOperator in a way such that calling torch.cat on a list of LinearOperator automatically creates a CatLinearOperator

[Max Balandat]

Yes, that makes a lot of sense and would be great to have. I'm not sure if that is easy to do with the __torch_function__ setup that we leverage for doing this dispatching under the hood. Let me see if I can get some intel on this.