[Prompty] AsyncFlow.load on prompty should return AsyncPrompty

[lalala123123]

Description

obj = AsyncFlow.load(source="path/to/prompty.prompty")

obj type should be AsyncPrompty.

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.
• Create an issue and link to the pull request to get dedicated review from promptflow team. Learn more: 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.

Testing Guidelines

• Pull request includes test coverage for the included changes.

[github-actions]

promptflow SDK CLI Azure E2E Test Result zhrua/fix_async_prompty

  4 files    4 suites   4m 18s ⏱️
243 tests 203 ✅  40 💤 0 ❌
972 runs  812 ✅ 160 💤 0 ❌

Results for commit 1325ac1.

♻️ This comment has been updated with latest results.

[github-actions]

promptflow-core test result

0 tests   0 ✅  0s ⏱️
0 suites  0 💤
0 files    0 ❌

Results for commit 1325ac1.

♻️ This comment has been updated with latest results.

[github-actions]

SDK CLI Global Config Test Result zhrua/fix_async_prompty

6 tests   6 ✅  1m 17s ⏱️
1 suites  0 💤
1 files    0 ❌

Results for commit 1325ac1.

♻️ This comment has been updated with latest results.

[github-actions]

Executor Unit Test Result zhrua/fix_async_prompty

796 tests   796 ✅  3m 45s ⏱️
  1 suites    0 💤
  1 files      0 ❌

Results for commit 1325ac1.

♻️ This comment has been updated with latest results.

[github-actions]

Executor E2E Test Result zhrua/fix_async_prompty

246 tests   240 ✅  5m 3s ⏱️
  1 suites    6 💤
  1 files      0 ❌

Results for commit 1325ac1.

♻️ This comment has been updated with latest results.

[github-actions]

SDK CLI Test Result zhrua/fix_async_prompty

    4 files      4 suites   1h 5m 58s ⏱️
  786 tests   763 ✅ 23 💤 0 ❌
3 144 runs  3 052 ✅ 92 💤 0 ❌

Results for commit 1325ac1.

♻️ This comment has been updated with latest results.