You need different output channels to apply softmax on them.
For example, if you would like to output a segmentation, where each channel stores the probabilities for one class, you would have [batch_size, n_class, w, h] as the output dimension.
Now you could call softmax on it and each pixel will have the probability belonging to the class corresponding to the channel, i.e. output[0, 0, ...] will give you a probability map of class one for every pixel.
If you only have one output channel, it seems you have a binary classification task?
In this case you could use sigmoid functions.