The default is (seq_len, batch_size, input_size) by default, but you can specify batch_first=True and use (batch_size, seq_len, input_size), so it is not a big problem unless you forget the parameter.
The reason for the default is that the RNN will iterate over the seq dimension, so it is efficient to access individual timestep, batched data, which is contiguous if you pass in contiguous tensors of form (seq_len, batch_size, input_size).
Best regards
Thomas