diff = source_embs.unsqueeze(-1) - target_embs.transpose(1,0).unsqueeze(0)

This line of code calculates the difference between two sets of embeddings: source_embs and target_embs.

- source_embs: a tensor of size (batch_size, embedding_dim), representing the embeddings for a batch of source inputs.
- target_embs: a tensor of size (batch_size, embedding_dim), representing the embeddings for a batch of target inputs.

The unsqueeze(-1) method adds a new dimension at the end of the tensor, resulting in a tensor of size (batch_size, embedding_dim, 1). This is done for both source_embs and target_embs.

The transpose(1,0) method swaps the first two dimensions of target_embs, resulting in a tensor of size (embedding_dim, batch_size). The unsqueeze(0) method then adds a new dimension at the beginning of the tensor, resulting in a tensor of size (1, embedding_dim, batch_size).

The two tensors are then broadcasted together to obtain a tensor of size (batch_size, embedding_dim, batch_size), where each element (i,j,k) represents the difference between the ith source embedding and the jth target embedding in the kth batch.