Fix relative promptflow-core import from sdk (#2510)

# 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.**
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- [ ] **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.

Signed-off-by: Brynn Yin <biyi@microsoft.com>

src/promptflow/promptflow/_sdk/_load_functions.py

@@ -7,9 +7,10 @@
 
 from dotenv import dotenv_values
 
-from .._utils.logger_utils import get_cli_sdk_logger
-from .._utils.yaml_utils import load_yaml
-from ..exceptions import UserErrorException
+from promptflow._utils.logger_utils import get_cli_sdk_logger
+from promptflow._utils.yaml_utils import load_yaml
+from promptflow.exceptions import UserErrorException
+
 from ._errors import MultipleExperimentTemplateError, NoExperimentTemplateError
 from .entities import Run
 from .entities._connection import CustomConnection, _Connection

src/promptflow/promptflow/_sdk/_pf_client.py

@@ -6,10 +6,11 @@
 from pathlib import Path
 from typing import Any, Dict, List, Union
 
-from .._constants import USER_AGENT_OVERRIDE_KEY, ConnectionProviderConfig
-from .._utils.logger_utils import get_cli_sdk_logger
-from .._utils.user_agent_utils import ClientUserAgentUtil, setup_user_agent_to_operation_context
-from ..exceptions import ErrorTarget, UserErrorException
+from promptflow._constants import USER_AGENT_OVERRIDE_KEY, ConnectionProviderConfig
+from promptflow._utils.logger_utils import get_cli_sdk_logger
+from promptflow._utils.user_agent_utils import ClientUserAgentUtil, setup_user_agent_to_operation_context
+from promptflow.exceptions import ErrorTarget, UserErrorException
+
 from ._configuration import Configuration
 from ._constants import MAX_SHOW_DETAILS_RESULTS
 from ._load_functions import load_flow

src/promptflow/promptflow/contracts/flow.py

@@ -10,14 +10,14 @@
 from pathlib import Path
 from typing import Any, Dict, List, Optional
 
+from promptflow._constants import LANGUAGE_KEY, FlowLanguage, MessageFormatType
+from promptflow._utils.utils import _match_reference, _sanitize_python_variable_name, try_import
 from promptflow._utils.yaml_utils import load_yaml
 from promptflow.contracts._errors import FlowDefinitionError
 from promptflow.exceptions import ErrorTarget
 from promptflow.tracing._utils import serialize
 
-from .._constants import LANGUAGE_KEY, FlowLanguage, MessageFormatType
 from .._sdk._constants import DEFAULT_ENCODING
-from .._utils.utils import _match_reference, _sanitize_python_variable_name, try_import
 from ._errors import FailedToImportModule
 from .tool import ConnectionType, Tool, ToolType, ValueType