When you are using datastore inputs for augmentedImageDatastore, imds and pxds cannot be provided as separate inputs to the function. There are two ways to specify the image augmentation methods:
1) You can create a pixelLabelImageDatastore object and provide that as an input.
ds = pixelLabelImageDatastore(imds,pxds);
augimds = augmentedImageDatastore(imageSize,imds,pxds,'DataAugmentation',imageAugmenter);
Please refer to the documentation page of augmentedImageDatastore where it explains the valid syntaxes.
augpxds = pixelLabelImageDatastore(imds,pxds,'DataAugmentation',imageAugmenter);
To answer your second question, splitEachLabel works only on imageDatastore as it is described in the documentation page.
Alternatively, you can use the partitionByIndex method provided by the pixelLabelImageDatastore object to split your data.
totalImages = numel(ds.Images);
% Split 80% of the data into training and rest into validation
idx = floor(0.8 * totalImages);
% Partition the datastore according to the specified indices
pximds_train = partitionByIndex(ds, 1:idx);
pximds_val = partitionByIndex(ds, (idx + 1):totalImages);
