nn.BCELoss expects the model output and target to have the same shape.
Based on your code snippet it seems that the model output has two additional dimensions, so could you explain what the model is supposed to predict and how this would fit the target shape?
Also, use torch.sigmoid, as nn.BCELoss expects probabilities coming from the model or (better) remove F.log_softmax and use nn.BCEWithLogitsLoss for more numerical stability.