If I understand the use case correctly, you would like to load all samples for a specific fix value?
If that’s the case you could either use a custom sampler, which returns batch samples sorted by the fix values. Alternatively, you could probably use different datasets for each fix and load the batches from these datasets as needed.
Is your dataset containing the same amount for each fix value? If not, this would mean that the batch size would be different, but I assume this fits your use case?