fix Number of Frames parameter

now users can't edit Number of Frames, probably because config.validation_data.video_length was always set to 8

gradio_demo/runner.py

@@ -105,7 +105,7 @@ class Runner:
         config.input_data.sample_frame_rate = sample_frame_rate
 
         config.validation_data.prompts = [validation_prompt]
-        config.validation_data.video_length = 8
+        config.validation_data.video_length = n_sample_frames
         config.validation_data.width = resolution
         config.validation_data.height = resolution
         config.validation_data.num_inference_steps = 50