The first error is thrown in a batch norm layer, which cannot calculate the current batch statistics for 1 value per channel.
The input shape of an image tensor is expected to be [batch_size, channels, height, width]. In your case the activation is a single sample with 256 channels and one pixel.
To avoid this error you could either increase the spatial size of the initial input, increase the batch size alternatively, or remove some pooling operation from your model.