'Prepares a data batch for forward.
Parameters
data_batch : DataBatch'

def prepare(self, data_batch):

    assert (self.binded and self.params_initialized)
    bucket_key = data_batch.bucket_key
    original_bucket_key = self._curr_bucket_key
    data_shapes = data_batch.provide_data
    label_shapes = data_batch.provide_label
    self.switch_bucket(bucket_key, data_shapes, label_shapes)
    self.switch_bucket(original_bucket_key, None, None)
