Multi-task learning in fastai v2 (joint regression and categorization from images)

I am working on a multitask problem, where I feed in an image (in this case, multi-spectral, but not super important) and receive both regression and categorization outputs. However, I’m hitting roadbumps and am curious if people have had success. I think this is very similar to the multi-task problem that was solved by @yang-zhang for fasta v1, but I haven’t seen a fastai v2 solution.

To do this, I created my DataBlock that expects an Image-like input, and then (thanks to n_inp=1) yields two outputs: a RegressionBlock and a CategoryBlock:

our_datablocks = (MSTensorImage(x = 12), RegressionBlock, CategoryBlock)

db = DataBlock(blocks = our_datablocks,
               get_items = find_image_zip_files,
               get_x = get_x_fn,
               get_y = (get_y_fn_age, get_y_fn_disease),
               n_inp=1 # Declare that only the first datablock is input
dls = db.dataloaders(source = path/'', bs = 2)
Collecting items ...
Found 20 items
2 datasets of sizes 16,4
Setting up Pipeline: get_x_fn
Setting up Pipeline: get_y_fn_age -> RegressionSetup -- {'c': None}
Setting up Pipeline: get_y_fn_disease -> Categorize -- {'vocab': None, 'sort': True, 'add_na': False}

So far, so good.

But creating the model itself, the problem becomes apparent:

learn = cnn_learner(dls = dls, 
                     arch = resnet18, 
                     loss_func = (MSELossFlat(), CrossEntropyLossFlat()),
                     normalize = False,
                     pretrained = True)

Clearly, unless fastai v2 knows what I mean by that list of loss functions and does a lot of work behind the scenes in cnn_learner, that’s not going to work. And sure enough, it does not:

TypeError                                 Traceback (most recent call last)
<ipython-input-91-e1fdee3ccda7> in <module>
----> 1 learn = cnn_learner(dls = dls, 
      2                      arch = resnet18,
      3                      n_in=12,
      4                      loss_func = (MSELossFlat(), CrossEntropyLossFlat()),
      5                      normalize = False,

/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/site-packages/fastai/vision/ in cnn_learner(dls, arch, normalize, n_out, pretrained, config, loss_func, opt_func, lr, splitter, cbs, metrics, path, model_dir, wd, wd_bn_bias, train_bn, moms, **kwargs)
    178     if n_out is None: n_out = get_c(dls)
    179     assert n_out, "`n_out` is not defined, and could not be inferred from data, set `dls.c` or pass `n_out`"
--> 180     model = create_cnn_model(arch, n_out, pretrained=pretrained, **kwargs)
    182     splitter=ifnone(splitter, meta['split'])

/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/site-packages/fastai/vision/ in create_cnn_model(arch, n_out, pretrained, cut, n_in, init, custom_head, concat_pool, **kwargs)
    144     if custom_head is None:
    145         nf = num_features_model(nn.Sequential(*body.children()))
--> 146         head = create_head(nf, n_out, concat_pool=concat_pool, **kwargs)
    147     else: head = custom_head
    148     model = nn.Sequential(body, head)

/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/site-packages/fastai/vision/ in create_head(nf, n_out, lin_ftrs, ps, concat_pool, first_bn, bn_final, lin_first, y_range)
     87     if lin_first: layers.append(nn.Dropout(ps.pop(0)))
     88     for ni,no,bn,p,actn in zip(lin_ftrs[:-1], lin_ftrs[1:], bns, ps, actns):
---> 89         layers += LinBnDrop(ni, no, bn=bn, p=p, act=actn, lin_first=lin_first)
     90     if lin_first: layers.append(nn.Linear(lin_ftrs[-2], n_out))
     91     if bn_final: layers.append(nn.BatchNorm1d(lin_ftrs[-1], momentum=0.01))

/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/site-packages/fastai/ in __init__(self, n_in, n_out, bn, p, act, lin_first)
    169         layers = [BatchNorm(n_out if lin_first else n_in, ndim=1)] if bn else []
    170         if p != 0: layers.append(nn.Dropout(p))
--> 171         lin = [nn.Linear(n_in, n_out, bias=not bn)]
    172         if act is not None: lin.append(act)
    173         layers = lin+layers if lin_first else layers+lin

/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/site-packages/torch/nn/modules/ in __init__(self, in_features, out_features, bias)
     76         self.in_features = in_features
     77         self.out_features = out_features
---> 78         self.weight = Parameter(torch.Tensor(out_features, in_features))
     79         if bias:
     80             self.bias = Parameter(torch.Tensor(out_features))

TypeError: new() received an invalid combination of arguments - got (L, int), but expected one of:
 * (*, torch.device device)
      didn't match because some of the arguments have invalid types: (L, int)
 * (torch.Storage storage)
 * (Tensor other)
 * (tuple of ints size, *, torch.device device)
 * (object data, *, torch.device device)

So I think that the next step is to create a custom head and a custom loss function. However, I’m curious if this is already a solved problem and someone has succeeded here already.

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@jamesp did you find any solution for the above problem? I am stuck at the exact same problem.

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Unfortunately, I did not. My attention got pulled away by other projects.