PyTorch: DecoderRNN: RuntimeError: input must have 3 dimensions, got 2
I am building a DecoderRNN using PyTorch (This is an image-caption decoder):

class DecoderRNN(nn.Module):
    def __init__(self, embed_size, hidden_size, vocab_size):

        super(DecoderRNN, self).__init__()
        self.hidden_size = hidden_size
        self.gru = nn.GRU(embed_size, hidden_size, hidden_size)
        self.softmax = nn.LogSoftmax(dim=1)

    def forward(self, features, captions):

        print (features.shape)
        print (captions.shape)
        output, hidden = self.gru(features, captions)
        output = self.softmax(self.out(output[0]))
        return output, hidden 


The data have the following shapes:

torch.Size([10, 200])  &lt;- features.shape (10 for batch size)
torch.Size([10, 12])   &lt;- captions.shape (10 for batch size)


Then I got the following errors. Any ideas what I missed here? Thanks!

---------------------------------------------------------------------------
RuntimeError                              Traceback (most recent call last)
&lt;ipython-input-2-76e05ba08b1d&gt; in &lt;module&gt;()
     44         # Pass the inputs through the CNN-RNN model.
     45         features = encoder(images)
---&gt; 46         outputs = decoder(features, captions)
     47 
     48         # Calculate the batch loss.

/opt/conda/lib/python3.6/site-packages/torch/nn/modules/module.py in __call__(self, *input, **kwargs)
    323         for hook in self._forward_pre_hooks.values():
    324             hook(self, input)
--&gt; 325         result = self.forward(*input, **kwargs)
    326         for hook in self._forward_hooks.values():
    327             hook_result = hook(self, input, result)

/home/workspace/model.py in forward(self, features, captions)
     37         print (captions.shape)
     38         # features = features.unsqueeze(1)
---&gt; 39         output, hidden = self.gru(features, captions)
     40         output = self.softmax(self.out(output[0]))
     41         return output, hidden

/opt/conda/lib/python3.6/site-packages/torch/nn/modules/module.py in __call__(self, *input, **kwargs)
    323         for hook in self._forward_pre_hooks.values():
    324             hook(self, input)
--&gt; 325         result = self.forward(*input, **kwargs)
    326         for hook in self._forward_hooks.values():
    327             hook_result = hook(self, input, result)

/opt/conda/lib/python3.6/site-packages/torch/nn/modules/rnn.py in forward(self, input, hx)
    167             flat_weight=flat_weight
    168         )
--&gt; 169         output, hidden = func(input, self.all_weights, hx)
    170         if is_packed:
    171             output = PackedSequence(output, batch_sizes)

/opt/conda/lib/python3.6/site-packages/torch/nn/_functions/rnn.py in forward(input, *fargs, **fkwargs)
    383             return hack_onnx_rnn((input,) + fargs, output, args, kwargs)
    384         else:
--&gt; 385             return func(input, *fargs, **fkwargs)
    386 
    387     return forward

/opt/conda/lib/python3.6/site-packages/torch/autograd/function.py in _do_forward(self, *input)
    326         self._nested_input = input
    327         flat_input = tuple(_iter_variables(input))
--&gt; 328         flat_output = super(NestedIOFunction, self)._do_forward(*flat_input)
    329         nested_output = self._nested_output
    330         nested_variables = _unflatten(flat_output, self._nested_output)

/opt/conda/lib/python3.6/site-packages/torch/autograd/function.py in forward(self, *args)
    348     def forward(self, *args):
    349         nested_tensors = _map_variable_tensor(self._nested_input)
--&gt; 350         result = self.forward_extended(*nested_tensors)
    351         del self._nested_input
    352         self._nested_output = result

/opt/conda/lib/python3.6/site-packages/torch/nn/_functions/rnn.py in forward_extended(self, input, weight, hx)
    292             hy = tuple(h.new() for h in hx)
    293 
--&gt; 294         cudnn.rnn.forward(self, input, hx, weight, output, hy)
    295 
    296         self.save_for_backward(input, hx, weight, output)

/opt/conda/lib/python3.6/site-packages/torch/backends/cudnn/rnn.py in forward(fn, input, hx, weight, output, hy)
    206         if (not is_input_packed and input.dim() != 3) or (is_input_packed and input.dim() != 2):
    207             raise RuntimeError(
--&gt; 208                 'input must have 3 dimensions, got {}'.format(input.dim()))
    209         if fn.input_size != input.size(-1):
    210             raise RuntimeError('input.size(-1) must be equal to input_size. Expected {}, got {}'.format(

RuntimeError: input must have 3 dimensions, got 2


Your GRU input needs to be 3-dimensional:


  input of shape (seq_len, batch, input_size): tensor containing the features of the input sequence.


Further you need to provide the hidden state (last encoder hidden state in this case) as second parameter:

self.gru(input, h_0)


Where input is your actual input and h_0 the hidden state which needs to be 3-dimensional as well:


  h_0 of shape (num_layers * num_directions, batch, hidden_size): tensor
  containing the initial hidden state for each element in the batch.
  Defaults to zero if not provided.


https://pytorch.org/docs/master/nn.html#torch.nn.GRU
