fix: fix SentenceTransformerRerank init

[lainconn]

Description

Fixes:

• SentenceTransformerRerank init with device equals to None except a valid value provided.
• Depreciated warning during CrossEncoder init.
• Added docstring for better readability.

Current implementation does not work unless you explicitly provide a class with device value, which is bad practice in general. Good practice is to set device to None and infer the device in class body.

I had to add sentence_transformers package to pyproject.toml due to unit tests.

Fixes # (issue)

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

Version Bump?

Did I bump the version in the pyproject.toml file of the package I am updating? (Except for the llama-index-core 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 uv run make format; uv run make lint to appease the lint gods

[AstraBert]

Seems like a reasonable change, thanks!

[lainconn]

@AstraBert Could you approve tests, please.

[lainconn]

@AstraBert Hello, any merge plans?

[AstraBert]

Hey there @lainconn
It's merged now, sorry for the delay😅