//   contracting_axes)
//   3. batch_dimensions = Gather(operand_shape, batch_axes)
//   4. flattened_shape = Concat(batch_dimensions, flattened_out_dim,
//   flattened_contracting_dim)
// The flattened shape for LHS
// is like [batch_dimensions, flattened_out_dimension,
// flattened_contracting_dimension] and [batch_dimensions,
// flattened_contracting_dimension, flattened_out_dimension] for RHS.