[tests] expand flex-attn test for vision models (#38434)

* expand the test for VLMs

* typo

* mark models `supports_flex` + expand test for additional kwargs

* flex attn for refactored vision models

* fix copies

* fix

* unskip

* style

* address comments

tests/models/gemma3/test_modeling_gemma3.py

@@ -351,12 +351,6 @@ class Gemma3Vision2TextModelTest(ModelTesterMixin, GenerationTesterMixin, unitte
     def test_initialization(self):
         pass
 
-    @unittest.skip(
-        reason="Siglip has no FLEX attention, and we don't have a proper way to set/test attn in VLMs. TODO @raushan"
-    )
-    def test_flex_attention_with_grads(self):
-        pass
-
     def test_automodelforcausallm(self):
         """
         Regression test for #36741/#36917 -- make sure `AutoModelForCausalLM` works with a Gemma3 config, i.e. that