Calculate nb of dataset elements in RetrieveDataset

Refs https://redmine.teklia.com/issues/5686

The endpoint RetrieveDataset should expose the number of elements in each sets as :

"set_elements": {
   "train": 12,
   "test...": 1,
   "whatever": 10,
}

We could use a more generic format too (would require a frontend update):

"sets": [
  {
    "name": "train",
    "nb_elements": 12,
  },
...
]

Using annotate it should be pretty fast to get those counts.