[PromptFlow] Move import to the head of the file to avoid package references.

[Hhhilulu]

Description

Move import to the head of the file to avoid package references.

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.

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.

Testing Guidelines

• Pull request includes test coverage for the included changes.

[github-actions]

Executor Unit Test Result luluzuo/move-import-to-header

    2 files      2 suites   44s ⏱️
161 tests 161 ✔️ 0 💤 0 ❌
322 runs  321 ✔️ 1 💤 0 ❌

Results for commit 7616695.

[github-actions]

☂️ Python Coverage

current status: ✅

Overall Coverage

Lines	Covered	Coverage	Threshold	Status
10899	8301	76%	40%	🟢

New Files

No new covered files...

Modified Files

File	Coverage	Status
src/promptflow/promptflow/executor/_line_execution_process_pool.py	59%	🟢
TOTAL	59%	🟢

updated for commit: 7616695 by action🐍

[github-actions]

Executor E2E Test Result luluzuo/move-import-to-header

    2 files      2 suites   6m 56s ⏱️
  91 tests   89 ✔️ 2 💤 0 ❌
182 runs  176 ✔️ 6 💤 0 ❌

Results for commit 7616695.

[github-actions]

SDK CLI Test Result luluzuo/move-import-to-header

    2 files      2 suites   17m 52s ⏱️
207 tests 195 ✔️ 12 💤 0 ❌
414 runs  390 ✔️ 24 💤 0 ❌

Results for commit 7616695.