Remove <cat-toy> validation prompt from examples/textual_inversion/textual_inversion_sdxl.py (#6877)

Remove <cat-toy> validation prompt from textual_inversion_sdxl.py

The `<cat-toy>` validation prompt is a default choice for the example task in the README. But no other part of `textual_inversion_sdxl.py` references the cat toy and `textual_inversion.py` has a default validation prompt of `None` as well.

So bring `textual_inversion_sdxl.py` in line with `textual_inversion.py` and change default validation prompt to `None`

Author: mnoukhov

examples/textual_inversion/textual_inversion_sdxl.py

@@ -372,7 +372,7 @@ def parse_args():
     parser.add_argument(
         "--validation_prompt",
         type=str,
-        default="A <cat-toy> backpack",
+        default=None,
         help="A prompt that is used during validation to verify that the model is learning.",
     )
     parser.add_argument(