[DFlash] add num_timesteps property for parity with LLaDA2 / SDAR / IDLM

Mirrors the convention used elsewhere in diffusers discrete-diffusion
pipelines: callback consumers and progress-bar UIs read num_timesteps
to know how many steps remain. The natural value for DFlash is the
number of speculative-decode blocks.

Author: kashif

src/diffusers/pipelines/dflash/pipeline_dflash.py

@@ -95,6 +95,10 @@ def __init__(
             draft_model=draft_model, target_model=target_model, tokenizer=tokenizer, scheduler=scheduler
         )
 
+    @property
+    def num_timesteps(self):
+        return self._num_timesteps
+
     # --- Prompt encoding ---
 
     def _prepare_input_ids(
@@ -391,6 +395,7 @@ def _new_cache(cfg):
         start = num_input_tokens
         global_step = 0
         num_blocks = (max_length - num_input_tokens + block_size - 1) // block_size
+        self._num_timesteps = int(num_blocks)
 
         # 5. Block-wise speculative decoding loop
         block_progress_bar_config = getattr(self, "_progress_bar_config", {}).copy()