While the batch size can very, it doesn’t represent the temporal dimension, but just how many samples you are processing at once. If your seq_length is static, you are not working with a variable temporal dimension.
Make sure to permute the input to the expected dimensions. By default RNNs expect an input of [seq_len, batch_size, features]. With batch_first=True the input should be [batch_size, seq_len, features].