Pytorch Luong global attention: what is the shape of the alignment vector supposed to be?
You answered yourself [sequence length, 1] is correct assuming you work with a single sentence. (Or actually, the 1 dimension depends on implementation.)
In practice, the data is typically batched, so it will be [batch, sequence length 1]. This can be element-wise multiplied with the encoder states of dimension [batch, sequence length, hidden size] and summed in the middle dimension to get a context vector of dimension [batch, hidden_size].
Note also that if the sentences do not have the same length, you need to deal with padding, i.e., set the padded positions to -inf before softmax.