help, how to decode png byte numpy array that with shape [batch, length, image_byte]

Hello, I am currently try to use external source method to read videos that are stored as a sequence of image frames (fixed length). However, it looks like the fn.decoders.image function only allows to decode data with shape [batch, image_byte]. Is there any way that I can modify the shape from [batch, length, image_byte] to [batch * length, image_byte] in define_graph section? Currently I cannot modify the ExternalSourceIterator class so I can only obtain png byte numpy array that with shape [batch, length, image_byte] and then I cannot find way to reshape data in nvidia.dali.pipeline.DataNode in define_graph function