Improve error message when incompatible torchvision / PyTorch are used

[fmassa]

A number of users report errors on the C++ operators when they update either PyTorch or torchvision, see for example #1916, with errors such as

RuntimeError: No such operator torchvision::nms

We should improve the error message and instead point out that their pytorch / torchvision versions are incompatible.
This will give a better user experience.

[pmeier]

Out of curiosity: does a list of compatible versions exist somewhere?

[fmassa]

Compatibility between PyTorch and torchvision versions is currently as follows:

• latest torchvision stable is compatible with latest pytorch stable
• latest torchvision nightly is compatible with latest pytorch nightly

So for nightlies, we could use the nightly date as a criteria for compatibility, although it should also work for users who are compiling torchvision from source (in which case, the compatibility constraints are much less restricted)

I agree that this is not ideal and might be a bit hard to track, but until we manage to find a way to make the PyTorch ABI fixed / stable (probably not going to happen), this will maybe be the only way.

I would love to hear other ideas though.

[anibali]

@fmassa I've found that it is currently very difficult to determine which pairings of PyTorch and Torchvision are compatible. For example, a question that I've had a few times in the past is "given that I have some old version PyTorch #.#.# installed (that I can't upgrade), what is the newest Torchvision that I can install with it?". Currently this is actually not so easy to find out. This is particularly important for a project like PyTorch, since there are numerous reasons why you may not be able to upgrade (eg API changes, Python 2 support, CUDA version support).

latest torchvision stable is compatible with latest pytorch stable

This answer is somewhat unhelpful for two reasons:

1. It's hard to track for old versions. The best I've been able to do is look up historical release dates from both projects and try to link them up, which is clearly not ideal. It doesn't even seem like the release notes indicate which PyTorch version is required for each release. A compatibility table in the README or something similar would address this point.
2. It still doesn't indicate what the minimum PyTorch version required for a particular Torchvision release is. For example, the current README seems to suggest that Torchvision 0.6.0 is compatible with PyTorch 1.4 ("TorchVision requires PyTorch 1.4 or newer"). But at the same time, setup.py seems to pin a particular version (although this can't be determined by looking at that file...):

   vision/setup.py

   Lines 65 to 67 in 222a599

   	pytorch_dep = 'torch'
   	if os.getenv('PYTORCH_VERSION'):
   	pytorch_dep += "==" + os.getenv('PYTORCH_VERSION')

[pmeier]

For example, a question that I've had a few times in the past is "given that I have some old version PyTorch #.#.# installed (that I can't upgrade), what is the newest Torchvision that I can install with it?". Currently this is actually not so easy to find out.

I second this. Had to do this multiple times myself.

If I'm not mistaken there is a one-to-one relation for the stable releases. If that is the case couldn't we put simple table or something similar somewhere?

[fmassa]

I agree that having a compatibility table would be a good start. But it would also help to raise informative error messages at runtime if the versions are incompatible.
It could even be as simple as grepping for symbol not found in the error message when loading the .so library, as it would probably cover most of the cases I think

[vincentqb]

Compatibility between PyTorch and torchvision versions is currently as follows:

• latest torchvision stable is compatible with latest pytorch stable
• latest torchvision nightly is compatible with latest pytorch nightly

How about we simply look at the version of pytorch, and throw a warning when importing torchvision with an incompatible version of pytorch?

[fmassa]

This is a good start, but things get a bit more complicated with nightlies and when compiling from master.
I think we should try loading the library first, and if it fails identify from the error message the the versions are incompatible, maybe redirecting the users to the torchvision README which should contain a compatibility matrix of PyTorch and torchvision versions, like torchtext does.

[vincentqb]

Compatibility between PyTorch and torchvision versions is currently as follows:

• latest torchvision stable is compatible with latest pytorch stable
• latest torchvision nightly is compatible with latest pytorch nightly

The official statement is fairly straightforward for releases, and we could make a table to keep track of which torchvision version was targeting which pytorch version.

For releases, as I suggested above, we could also explicitly throw a warning when a different version is detected.

For nightlies or master, I see those as "at your own risk" with minimal guarantees. I would not say more than what the statement says above in the readme.

1. It's hard to track for old versions.

I agree. I believe a table would help with that.

2. It still doesn't indicate what the minimum PyTorch version required for a particular Torchvision release is.

The official statement implicitly talks of exact version matches, and we can make that more clear. Making torchvision support minimal pytorch version is a much bigger burden, and outside the scope of what I understand this issue to be.

@fmassa -- are you planning to extend the support of torchvision outside the official statement? AFAIK torchaudio and torchtext are not planning this.

This is a good start, but things get a bit more complicated with nightlies and when compiling from master.

Were users here having issues with nighltlies or master? Or when using conda/pip to install torchvision? Again, I see nightlies and master as "at your own risk" with minimal guarantees. torchvision should simply be compatible with the latest master at all time.

I think we should try loading the library first, and if it fails identify from the error message the the versions are incompatible, maybe redirecting the users to the torchvision README which should contain a compatibility matrix of PyTorch and torchvision versions

Incompatible versions can lead to failures in many unexpected ways, and detecting all of them can be tricky. Once we detect incompatible versions, what do we do? Do we stop the import? Do we return a warning? Or maybe we selectively disabling features that are not supported?

Catching errors and throwing other ones could also obscure legitimate issues. Thoughts?

like torchtext does.

What does torchtext do?

[anibali]

Were users here having issues with nighltlies or master? Or when using conda/pip to install torchvision? Again, I see nightlies and master as "at your own risk" with minimal guarantees. torchvision should simply be compatible with the latest master at all time.

Speaking for myself, I'm only really concerned with the release versions available via conda/pip. I don't foresee a situation in which I'd personally need to figure out the PyTorch version compatibilities for old Torchvision nightly builds.

[fmassa]

are you planning to extend the support of torchvision outside the official statement? AFAIK torchaudio and torchtext are not planning this.

No, we will only enforce to be compatible with latest release, if it ends up working with older releases (if compiled from source) that's a bonus, not a requirement.

Incompatible versions can lead to failures in many unexpected ways, and detecting all of them can be tricky. Once we detect incompatible versions, what do we do? Do we stop the import? Do we return a warning? Or maybe we selectively disabling features that are not supported?

My thinking was: import should not error. Users might not want to use C++ extensions, and the code should still hopefully work fine in most cases. But if the user calls into a C++ extension, then they should ideally get a good error message.

Catching errors and throwing other ones could also obscure legitimate issues. Thoughts?

Fair point. One of the issues we currently have is that in order to support torchhub, we need to catch an error at import and not hard-fail. But this makes error messages down the road a bit worse IMO.

What does torchtext do?

I was referring to a table like https://github.com/pytorch/text#installation

Speaking for myself, I'm only really concerned with the release versions available via conda/pip. I don't foresee a situation in which I'd personally need to figure out the PyTorch version compatibilities for old Torchvision nightly builds.

I agree, let's not overly complicate the problem. Having a compatibility matrix in the readme, and a better error message when running C++ ops that failed loading the .so pointing to the compatibility matrix should be enough I think

[pmeier]

I've went through the blog posts and release notes and compiled the following table:

torch	torchvision	python
master / nightly	master / nightly	>=3.6
1.5.0	0.6.0	>=3.5
1.4.0	0.5.0	==2.7, >=3.5, <=3.8
1.3.0	0.4.2	==2.7, >=3.5, <=3.7
1.2.0	0.4.0	==2.7, >=3.5, <=3.7
1.1.0	0.3.0	==2.7, >=3.5, <=3.7
1.0.0		==2.7, >=3.5, <=3.7
<=0.4.1		==2.7, >=3.5, <=3.7

• Can someone help me fill in the blanks? I couldn't find a note about torchvision in the release notes of torch==1.0.0 and torch==0.4.1. Furthermore, I couldn't find a release of torchvision that matched the release date of these torch versions.
• How do we handle the patch versions, i.e. 0.4.X. Per release notes is torchvision==0.4.1 a compatibility release for torch==1.3.0 and torchvision==0.5.0 corresponds to torch==1.4.0. How does torchvision==0.4.2 fit in here?
• Related to the above comment: How do we want to handle the patch versions of torch, i.e. 1.3.1, 1.4.1, and so on? Do we add all compatible versions, only the latest release or even only the first release in the table?