Use dict instead of OrderedDict in maxvit

[NicolasHug]

Related to #6925 (comment)

dicts are ordered from Python 3.6 3.7, so we might not need to use typing.OrderedDict which is causing failures with 3.7.0 and which is deprecated in 3.9.

[datumbox]

This dictionary was intended to maintain the order of layers so that the sequential call at like 469 will execute them in the right order.

Edit: think the order might be maintained for low number of keys but this is a not-guaranteed behaviour and might be different from platform to platform. I recommend restoring the OrderedDict here as this is the correct idiom and fix the typing annotations not to depend on the deprecated typing.OrderedDict.

[NicolasHug]

@datumbox all dicts preserve insertion order as of Python 3.6: https://stackoverflow.com/questions/39980323/are-dictionaries-ordered-in-python-3-6. EDIT: more importantly it is also a guaranteed language feature from 3.7, so it's not just a CPython detail.

I was assuming this would fulfil the requirements you noted for the sequential call. Am I missing something important?

[datumbox]

PyTorch Sequential accepts an OrderedDict according to the docs. Since Dict can in principle be any implementation of the data structure, it's best not to rely on the order of the keys which might change on the future again. Here is an alternative patch at #6929 which maintains OrderedDict, just fixes the typing info and passes mypy without extra ignores. I'll close that PR as it's meant only for illustration purpose but feel free to cherrypick some of the approach here.

[NicolasHug]

Since Dict can in principle be any implementation of the data structure

No, it's not an implementation detail anymore in 3.7. It's a guaranteed language feature (see link above). We can rely on it.

As noted below Sequential should be able to handle dicts on top of OrderedDicts now that 3.7 is the minimum required version. It might be a good opportunity to make that change upstream (looks like @pmeier is looking into it)

[oke-aditya]

Yes dicts are insertion ordered from python 3.7. This is legit feature and used widely.

[datumbox]

Same here, by removing the order the Sequential won't work as we expect.

[NicolasHug]

torchvision/models/maxvit.py:125: error: No overload variant of "Sequential"
matches argument type "Dict[str, Module]"  [call-overload]
            self.layers = nn.Sequential(_layers)

Sigh. I won't have too much time to dedicate to this. @pmeier could you please take over this PR to make Sequential happy? The goal here is to remove the use of the offending typing.OrderedDict.

This sounds like a limitation of Sequential BTW. It should accept pure dicts since they're guaranteed to be insertion-ordered. Might be worth fixing that upstream?

[datumbox]

@pmeier The following PR makes mypy happy without modifying the model: #6929

[NicolasHug]

Sequential doesn't support dicts for now, so I'll close this PR. Support request is tracked in pytorch/pytorch#84751