I try to implement a two-layer bidirectional LSTM with torch.nn.LSTM.
I made a toy example: a batch of 3 tensors, which are exactly the same (see my code below). And I expected the outputs of the BiLSTM  to be the the same along the batch dimension, i.e. out[:,0,:] == out[:,1,:] == out[:, 2, :].
But it seemed not to be the case. According to my experiments, 20%~40% of the time, the output were not the same. So I wonder where I got it wrong.
# Python 3.6.6, Pytorch 0.4.1
import torch

def test(hidden_size, in_size):
    seq_len, batch = 4, 3
    bilstm = torch.nn.LSTM(input_size=in_size, hidden_size=hidden_size, 
                            num_layers=2, bidirectional=True)

    # create a batch with 3 exactly the same tensors
    a = torch.rand(seq_len, 1, in_size)  # (seq_len, 1, in_size)
    x = torch.cat((a, a, a), dim=1)

    out, _ = bilstm(x)  # (seq_len, batch, n_direction * hidden_size)

    # expect the output should be the same along the batch dimension
    assert torch.equal(out[:, 0, :], out[:, 1, :])  
    assert torch.equal(out[:, 1, :], out[:, 2, :])

if __name__ == '__main__':
    count, total = 0, 0
    for h_size in range(1, 51):
        for in_size in range(1, 51):
            total += 1
            try:
                test(h_size, in_size)
            except AssertionError:
                count += 1
    print('percentage of assertion error:', count / total)