TST Add test for FSDP ignored_modules as str (#3719)

Follow up to #3698.

tests/fsdp/test_fsdp.py

@@ -398,6 +398,21 @@ class FSDPPluginIntegration(AccelerateTestCase):
             assert fsdp_plugin.cpu_ram_efficient_loading is False
             assert os.environ.get("FSDP_CPU_RAM_EFFICIENT_LOADING") == "False"
 
+    def test_ignored_modules_regex(self):
+        # Check that FSDP's ignored_modules can be a string, in which case it is treated as a regex
+        if self.current_fsdp_version != 1:
+            self.skipTest("ignored_modules only relevant for FSDP1")
+
+        env = self.fsdp_envs[1].copy()
+        env["FSDP_IGNORED_MODULES"] = ".*\\.q_proj$"
+        with patch_environment(**env):
+            accelerator = Accelerator()
+            model = AutoModel.from_pretrained(LLAMA_TESTING)
+            # model has 2 layers
+            layers_to_ignore = {model.layers[0].self_attn.q_proj, model.layers[1].self_attn.q_proj}
+            model = accelerator.prepare(model)
+            assert model._ignored_modules == layers_to_ignore
+
 
 @require_fsdp2
 @require_non_cpu