>>> embedding = nn.Embedding(10, 3, padding_idx=-1)
>>> tokens = torch.tensor([1, 2]).long()   # positive indexes here
>>> embedding(tokens)
tensor([[-0.6424, -1.2020,  0.4287],
        [ 0.5234,  1.2113,  0.8808]], grad_fn=<EmbeddingBackward>)