I guess 64 would be the batch size, while 76800 would probably be the temporal size (or the flattened batch dim and temp. dim)?
Anyway, I think you might want to consider using the last time step only or reducing the temporal dimension somehow (e.g. with a mean) to get an output of [batch_size, out_features].
I’m not familiar with your use case, so please feel free to add more information in case my suggestions wouldn’t work.