[prompty] load_flow(source=path/to/prompty) is callable (#3015)

# Description

Please add an informative description that covers that changes made by
the pull request and link all relevant issues.

# All Promptflow Contribution checklist:
- [ ] **The pull request does not introduce [breaking changes].**
- [ ] **CHANGELOG is updated for new features, bug fixes or other
significant changes.**
- [ ] **I have read the [contribution guidelines](../CONTRIBUTING.md).**
- [ ] **Create an issue and link to the pull request to get dedicated
review from promptflow team. Learn more: [suggested
workflow](../CONTRIBUTING.md#suggested-workflow).**

## General Guidelines and Best Practices
- [ ] Title of the pull request is clear and informative.
- [ ] There are a small number of commits, each of which have an
informative message. This means that previously merged commits do not
appear in the history of the PR. For more information on cleaning up the
commits in your PR, [see this
page](https://github.com/Azure/azure-powershell/blob/master/documentation/development-docs/cleaning-up-commits.md).

### Testing Guidelines
- [ ] Pull request includes test coverage for the included changes.

src/promptflow-devkit/promptflow/_sdk/entities/_flows/prompty.py

@@ -27,6 +27,7 @@ def __init__(
         path = Path(path)
         # prompty folder path
         code = Path(code)
+        self._core_prompty = CorePrompty(path=path, **kwargs)
         super().__init__(code=code, path=path, data=data, content_hash=None, **kwargs)
 
     @property
@@ -52,6 +53,9 @@ def _load(cls, path: Path, raise_error=True, **kwargs):
             )
         return cls(path=path, code=path.parent, data=data, **kwargs)
 
+    def __call__(self, *args, **kwargs):
+        return self._core_prompty(*args, **kwargs)
+
     # endregion overrides
 
     # region SchemaValidatableMixin

src/promptflow-devkit/tests/sdk_cli_test/e2etests/test_prompty.py

@@ -10,6 +10,7 @@
 from openai.types.chat import ChatCompletion
 
 from promptflow._sdk._pf_client import PFClient
+from promptflow.client import load_flow
 from promptflow.core import AsyncPrompty, Flow, Prompty
 from promptflow.core._errors import (
     InvalidConnectionError,
@@ -19,6 +20,7 @@
 )
 from promptflow.core._model_configuration import AzureOpenAIModelConfiguration
 from promptflow.core._prompty_utils import convert_model_configuration_to_connection
+from promptflow.exceptions import UserErrorException
 
 TEST_ROOT = PROMPTFLOW_ROOT / "tests"
 DATA_DIR = TEST_ROOT / "test_configs/datas"
@@ -171,6 +173,14 @@ def test_prompty_callable(self, pf: PFClient):
             Prompty.load(source=f"{PROMPTY_DIR}/prompty_example.prompty", model=model_dict)
         assert "Cannot configure model config and connection" in ex.value.message
 
+        prompty = load_flow(source=f"{PROMPTY_DIR}/prompty_example.prompty")
+        result = prompty(question="what is the result of 1+1?")
+        assert "2" in result
+
+        with pytest.raises(UserErrorException) as ex:
+            prompty("what is the result of 1+1?")
+        assert "Prompty can only be called with keyword arguments." in ex.value.message
+
     def test_prompty_async_call(self):
         async_prompty = AsyncPrompty.load(source=f"{PROMPTY_DIR}/prompty_example.prompty")
         with pytest.raises(MissingRequiredInputError) as e: