See nn_triplet_margin_with_distance_loss()
nnf_triplet_margin_with_distance_loss(
  anchor,
  positive,
  negative,
  distance_function = NULL,
  margin = 1,
  swap = FALSE,
  reduction = "mean"
)the anchor input tensor
the positive input tensor
the negative input tensor
(callable, optional): A nonnegative, real-valued function that
quantifies the closeness of two tensors. If not specified,
nn_pairwise_distance() will be used.  Default: None
Default: 1.
The distance swap is described in detail in the paper Learning shallow
convolutional feature descriptors with triplet losses by V. Balntas, E. Riba et al.
Default: FALSE.
(string, optional) – Specifies the reduction to apply to the output: 'none' | 'mean' | 'sum'. 'none': no reduction will be applied, 'mean': the sum of the output will be divided by the number of elements in the output, 'sum': the output will be summed. Default: 'mean'