You would have to decide which dimension(s) should be the temporal dimension (seq_len) and which the features (input_size).
E.g. you could treat the output channels as the features and the spatial dimensions (height and width) as the temporal dimension.
To do so, you could first flatten the spatial dimensions via:
output = output.view(output.size(0), output.size(1), -1)

and then permute the dimensions via:
output = output.permute(2, 0, 1)