refactor(API): improve usability

[zigzagcai]

Motivation

1. refactor API to easier usage
2. enhance support for FSDP with EP

Modification

T.B.A.

BC-breaking (Optional)

T.B.A.

Use cases (Optional)

T.B.A.

Checklist

Before PR:

• Pre-commit or other linting tools are used to fix the potential lint issues.
• Bug fixes are fully covered by unit tests, the case that causes the bug should be added in the unit tests.
• The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, like docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with those projects.
• CLA has been signed and all committers have signed the CLA in this PR.