docs: add missing docstrings to Dense methods and to_scipy_coo

[RavSinghChandan]

What does this PR do?

Adds missing docstrings to five public methods/functions across two files. Each docstring follows the existing HuggingFace-style documentation patterns used elsewhere in the codebase.

sentence_transformers/base/modules/dense.py — 4 methods:

Method	Added
Dense.forward	Args / Returns
Dense.get_embedding_dimension	Returns
Dense.get_config_dict	Returns
Dense.save	Args

The Dense class had a thorough class-level docstring but its individual methods were undocumented, making it harder to understand what each method expects and returns at a glance.

sentence_transformers/util/tensor.py — 1 function:

Function	Added
to_scipy_coo	Args / Returns / Example

to_scipy_coo sits next to compute_count_vector (which already has a full docstring) but had no documentation of its own.

Tests

No logic changed — documentation only.

ruff check and ruff format --check both pass. typos reports no issues.

[tomaarsen]

Hello!

Thanks for the PR. For the Dense methods: forward, get_config_dict, and save already inherit their docstrings from the base Module class, so they aren't really missing. They're also not rendered in the API reference, only the class docstring shows. Simpler modules like Pooling and Normalize intentionally leave these to the inherited base docstrings, and I'd like to keep Dense consistent with that. So I'd like to pass on the Dense docstrings.

On the other hand, to_scipy_coo is a nice addition that I'd be happy to add. Could you trim the PR down to just the to_scipy_coo docstring?

• Tom

[tomaarsen]

See above