Arbitrary `prev_action_emb` when pointer maps have multiple entries

[rshin]

For choosing a table in Spider, we allow the model to point to the embedding for any of its columns or the embedding for the table itself: https://github.com/rshin/seq2struct/blob/e69c8eb182ec80770cde4cd369e1b698bc8e921a/seq2struct/models/spider_enc.py#L321-L328

However, when computing the prev_action_emb, we only use the first of these embeddings (i.e. the first column of the table): https://github.com/rshin/seq2struct/blob/e69c8eb182ec80770cde4cd369e1b698bc8e921a/seq2struct/models/nl2code/decoder.py#L501-L516 https://github.com/rshin/seq2struct/blob/e69c8eb182ec80770cde4cd369e1b698bc8e921a/seq2struct/models/nl2code/decoder.py#L1263 https://github.com/rshin/seq2struct/blob/e69c8eb182ec80770cde4cd369e1b698bc8e921a/seq2struct/models/nl2code/decoder.py#L1465-L1468 https://github.com/rshin/seq2struct/blob/e69c8eb182ec80770cde4cd369e1b698bc8e921a/seq2struct/models/nl2code/decoder.py#L1442