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Computes the contrastive loss between y_true and y_pred.

This loss encourages the embedding to be close to each other for the samples of the same label and the embedding to be far apart at least by the margin constant for the samples of different labels.

The euclidean distances y_pred between two embedding matrices a and b with shape [batch_size, hidden_size] can be computed as follows:

# y_pred = \sqrt (\sum_i (a[:, i] - b[:, i])^2)
y_pred = tf.linalg.norm(a - b, axis=1)


y_true 1-D integer Tensor with shape [batch_size] of binary labels indicating positive vs negative pair.
y_pred 1-D float Tensor with shape [batch_size] of distances between two embedding matrices.
margin margin term in the loss definition.

contrastive_loss 1-D float Tensor with shape [batch_size].