feat: add stepfun integrations

[bestony]

Description

Add new LLM Vendor named "stepfun" for https://platform.stepfun.com/

New Package?

Did I fill in the tool.llamahub section in the pyproject.toml and provide a detailed README.md for my new integration or package?

• Yes
• No

Type of Change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Your pull-request will likely not be merged unless it is covered by some form of impactful unit testing.

• I added new unit tests to cover this change
• I believe this change is already covered by existing unit tests

Suggested Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added Google Colab support for the newly added notebooks.
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I ran make format; make lint to appease the lint gods

[bestony]

@masci
can you help me review my PR? i need at least 1 approving review for run CI check.

[bestony]

localhost run result :

sample code

from llama_index.llms.stepfun import StepFun
from llama_index.core.llms import ChatMessage

llm = StepFun(
    api_key="",
    max_tokens=256,
    context_window=4096,
    model="step-1v-8k",
)

message = ChatMessage(role="user", content="Tell me a joke")
resp = llm.chat([message])
print(resp)

[masci]

Left a couple of comments. Can you also fix the linting problems? If you install our pre-commit hook that should be super easy pre-commit install

[bestony]

done with pre-commit @masci