fix(train): align DDP no_sync with gradient accumulation (#3472)

Synchronize DDP/FSDP gradients on the final microbatch of each accumulation window.

Verified with a two-rank CPU/Gloo reproduction, py_compile, diff checks, and a clean synthetic merge against current main.

Author: YuCeong-May

funasr/train_utils/trainer_ds.py

@@ -648,7 +648,9 @@ def train_epoch(
 
             my_context = nullcontext
             if self.use_ddp or self.use_fsdp:
-                my_context = model.no_sync if batch_idx % accum_grad != 0 else my_context
+                my_context = (
+                    model.no_sync if (batch_idx + 1) % accum_grad != 0 else my_context
+                )
             with my_context():
                 time2 = time.perf_counter()