Make Gradio temp directory if it doesn't exist

Gradio normally creates the temp directory in `pil_to_temp_file()` (861d752a83/gradio/components/base.py (L313)) but since the Gradio implementation of `pil_to_temp_file()` is replaced with `save_pil_to_file()`, the Gradio temp directory should also be created by `save_pil_to_file()` when necessary.

modules/ui_tempdir.py

@@ -44,6 +44,8 @@ def save_pil_to_file(self, pil_image, dir=None, format="png"):
 
     if shared.opts.temp_dir != "":
         dir = shared.opts.temp_dir
+    else:
+        os.makedirs(dir, exist_ok=True)
 
     use_metadata = False
     metadata = PngImagePlugin.PngInfo()