Most likely you have a nn.BatchNorm layer somewhere in your model, which expects more then 1 value to calculate the running mean and std of the current batch.
In case you want to validate your data, call model.eval() before feeding the data, as this will change the behavior of the BatchNorm layer to use the running estimates instead of calculating them for the current batch.
If you want to train your model and can’t use a bigger batch size, you could switch e.g. to InstanceNorm.