Fix NaN from stale FP4 scale padding in create_fp4_scale_tensor (#38148)

Signed-off-by: Elvir Crncevic <elvircrn@gmail.com>
Co-authored-by: Tyler Michael Smith <tyler@neuralmagic.com>

vllm/_custom_ops.py

@@ -56,11 +56,11 @@ def create_fp4_scale_tensor(
         rounded_m = round_up(m, 128)
         scale_n = n // block_size
         rounded_n = round_up(scale_n, 4)
-        return torch.empty(
+        return torch.zeros(
             (rounded_m, rounded_n // 4), device=device, dtype=torch.int32
         )
     else:
-        return torch.empty((m, n // block_size), device=device, dtype=torch.uint8)
+        return torch.zeros((m, n // block_size), device=device, dtype=torch.uint8)
 
 
 def create_fp4_output_tensors(