message: Use balanced sharding strategy in bigquery read session.
  We use sharding to provide a way of shuffling the read order. With liquid sharding, a single stream can read all data which wouldn't be shuffled.
diff: @@ -149,6 +149,7 @@ class BigQueryReadSessionOp : public OpKernel {
  createReadSessionRequest.mutable_read_options()->set_row_restriction(
  row_restriction_);
  createReadSessionRequest.set_requested_streams(requested_streams_);
  + createReadSessionRequest.set_sharding_strategy(apiv1beta1::ShardingStrategy::BALANCED);
  createReadSessionRequest.set_format(apiv1beta1::DataFormat::AVRO);
  VLOG(3) << "createReadSessionRequest: "
  << createReadSessionRequest.DebugString();
  