refactor: use DecoderLayer pattern; drop moe_latent_size; inherit MixtralExperts
- Replace Block/mixer abstraction with NemotronH{Dense,Sparse}DecoderLayer
holding exactly one of self.mamba / self.self_attn / self.mlp|block_sparse_moe
based on layer_type, matching the standard decoder-layer pattern.
- Pass attention_mask (4D causal) + mamba_attention_mask (raw 2D) uniformly
to every layer instead of pre-dispatching per layer type.
- NemotronHSparseExperts now inherits from MixtralExperts (non-gated variant).
- Drop moe_latent_size from sparse config and the matching latent projections —
the field is not present in the released Nemotron-3 config.