Fix offset in LinearEmbeddingEncoder (#455)

This PR fixes an issue in the `LinearEmbeddingEncoder` class. The
forward function does not increment the `start_idx` variable, so only
the first one is used correctly if multiple embedding columns are
present.

This probably affects the results in the
[RelBench](https://arxiv.org/pdf/2407.20060) paper, so I suggest
double-checking those.

---------

Co-authored-by: Akihiro Nitta <nitta@akihironitta.com>

torch_frame/nn/encoder/stype_encoder.py

@@ -732,6 +732,7 @@ def encode_forward(
             # -> [batch_size, out_channels]
             x_lin = feat.values[:, start_idx:end_idx] @ self.weight_list[idx]
             x_lins.append(x_lin)
+            start_idx = end_idx
         # [batch_size, num_cols, out_channels]
         x = torch.stack(x_lins, dim=1)
         # [batch_size, num_cols, out_channels] + [num_cols, out_channels]