[QoL] Allow dtype str for torch_dtype arg of from_pretrained (#31590)

* Allow dtype str for torch_dtype in from_pretrained

* Update docstring

* Add tests for str torch_dtype

tests/test_modeling_utils.py

@@ -445,6 +445,18 @@ class ModelUtilsTest(TestCasePlus):
         with self.assertRaises(ValueError):
             model = AutoModel.from_config(config, torch_dtype=torch.int64)
 
+    def test_model_from_config_torch_dtype_str(self):
+        # test that from_pretrained works with torch_dtype being strings like "float32" for PyTorch backend
+        model = AutoModel.from_pretrained(TINY_T5, torch_dtype="float32")
+        self.assertEqual(model.dtype, torch.float32)
+
+        model = AutoModel.from_pretrained(TINY_T5, torch_dtype="float16")
+        self.assertEqual(model.dtype, torch.float16)
+
+        # torch.set_default_dtype() supports only float dtypes, so will fail with non-float type
+        with self.assertRaises(ValueError):
+            model = AutoModel.from_pretrained(TINY_T5, torch_dtype="int64")
+
     def test_model_from_pretrained_torch_dtype(self):
         # test that the model can be instantiated with dtype of either
         # 1. explicit from_pretrained's torch_dtype argument