Examples:
  >>> triplet_loss = nn.TripletMarginLoss(margin=1.0, p=2)
  >>> anchor = torch.randn(100, 128, requires_grad=True)
  >>> positive = torch.randn(100, 128, requires_grad=True)
  >>> negative = torch.randn(100, 128, requires_grad=True)
  >>> output = triplet_loss(anchor, positive, negative)
  >>> output.backward()
