fix save_weights_fast_with_mtp

[LiuXTao]

This PR includes two fixes:

1. The first fix addresses an issue where, when MTP is enabled, save_weights_fast did not correctly handle the duplicated MTP embedding. As a result, the layer with this name was saved twice, which triggered the shard count check failure.;

2. the other fixes an MTP construction error triggered when pp > 0 and num_layers_in_first_pipeline_stage is set.