Update pipeline_onnx_stable_diffusion.py with comment for previous commit

Added comment on purpose of init_noise_sigma.  This comment exists in related scripts that use the same line of code, but it was missing here.

Author: lostdisc

src/diffusers/pipelines/stable_diffusion/pipeline_onnx_stable_diffusion.py

@@ -383,6 +383,7 @@ def __call__(
         # set timesteps
         self.scheduler.set_timesteps(num_inference_steps)
 
+        # scale the initial noise by the standard deviation required by the scheduler
         latents = latents * self.scheduler.init_noise_sigma
 
         # prepare extra kwargs for the scheduler step, since not all schedulers have the same signature