Adds an assertion that "p.weight_split_dims_mapping" must be specified when p.device_mesh is specified for MultiHeadedProjectionLayer.
Adds an assertion that "p.weight_split_dims_mapping" must be specified when p.device_mesh is specified for MultiHeadedProjectionLayer.

PiperOrigin-RevId: 352327860

tensorflow/lingvo,tensorflow/lingvo,tensorflow/lingvo,tensorflow/lingvo