One way is to dig into Tensorflow code to see where is defined the list of POS and then import it to use in your code. You can find the list of the POS in the Github code of tensorflow Datasets there (UPOS constant): https://github.com/tensorflow/datasets/blob/master/tensorflow_datasets/core/dataset_builders/conll/conllu_dataset_builder_utils.py#L31

The item order is their index so with display(pd.Series(UPOS)), you get:

Another way would be to extract the items from the upos column of tfds.as_dataframe (taking a few rows, concatenating the upos values, splitting by the separating character and taking the set() to get the unique values.