[https://nvbugs/5781589][fix] Skip spec dec for non-last rank (#10445)

Signed-off-by: ziyixiong-nv <219238287+ziyixiong-nv@users.noreply.github.com>

Author: ziyixiong-nv

tensorrt_llm/_torch/models/modeling_speculative.py

@@ -953,6 +953,14 @@ def forward(
             hidden_states = hidden_states[:attn_metadata.num_tokens]
 
         if self.draft_model is not None:
+            # For one-model speculative decoding with PP, only the last PP rank
+            # has valid hidden_states from the target model. The spec_worker (which
+            # runs the draft model loop) should only run on the last PP rank.
+            # Non-last PP ranks return None and let the PP sync handle the results.
+            mapping = self.model.model_config.mapping
+            if mapping.has_pp() and not mapping.is_last_pp_rank():
+                return None
+
             # get logits
             logits = self.logits_processor.forward(
                 hidden_states[spec_metadata.gather_ids],