docs: def build_vocab(self, *args, **kwargs):
          """Construct the Vocab object for nesting field and combine it with this field's vocab.
  
          Arguments:
              Positional arguments: Dataset objects or other iterable data
                  sources from which to construct the Vocab object that
                  represents the set of possible values for the nesting field. If
                  a Dataset object is provided, all columns corresponding
                  to this field are used; individual columns can also be
                  provided directly.
              Remaining keyword arguments: Passed to the constructor of Vocab.
          """
          sources = []
          for arg in args:
              if isinstance(arg, Dataset):
                  sources.extend(
                      [getattr(arg, name) for name, field in arg.fields.items()
                       if field is self]
                  )
              else:
                  sources.append(arg)
  
          flattened = []
          for source in sources:
              flattened.extend(source)
          old_vectors = None
          old_unk_init = None
          old_vectors_cache = None
          if "vectors" in kwargs.keys():
              old_vectors = kwargs["vectors"]
              kwargs["vectors"] = None
          if "unk_init" in kwargs.keys():
              old_unk_init = kwargs["unk_init"]
              kwargs["unk_init"] = None
          if "vectors_cache" in kwargs.keys():
              old_vectors_cache = kwargs["vectors_cache"]
              kwargs["vectors_cache"] = None
          # just build vocab and does not load vector
          self.nesting_field.build_vocab(*flattened, **kwargs)
          super(NestedField, self).build_vocab()
          self.vocab.extend(self.nesting_field.vocab)
          self.vocab.freqs = self.nesting_field.vocab.freqs.copy()
          if old_vectors is not None:
              self.vocab.load_vectors(old_vectors,
                                      unk_init=old_unk_init, cache=old_vectors_cache)
  
          self.nesting_field.vocab = self.vocab