[python-package] add type hints on Dataset constructors

[jameslamb]

Contributes to #3756.
Contributes to #3867.

Adds type hints on some methods in Dataset which don't currently have them.

Notes for Reviewers

I'm beginning work on the "array-like" inputs in LightGBM's Python API, per #3756 (comment).

This is a first step towards that.

[jmoralez]

WDYT about defining a type for the dataset handle like in

LightGBM/include/LightGBM/c_api.h

Line 29 in 0a5c583

typedef void* DatasetHandle; /*!< \brief Handle of dataset. */

[jameslamb]

WDYT about defining a type for the dataset handle like in

Can you be more specific about what you're proposing for the Python side and what the benefit would be? I don't understand.

[jmoralez]

Something like:

DatasetHandle = ctypes.c_void_p

def __init_from_np2d(
    self,
    mat: np.ndarray,
    params_str: str,
    ref_dataset: Optional[DatasetHandle]
) -> "Dataset":

The benefit in my opinion is that when reading the code instead of reading: "the reference dataset is a void pointer", you'll instead read: "the reference dataset is a dataset handle"

[jameslamb]

ah ok, so just for documentation purposes? Yeah that works for me! I'll make that change here.

[jameslamb]

updated in 00b4477

thanks for the suggestion!

[jmoralez]

Thanks!

[jameslamb]

😬

This is a scheduled macOS-10.15 brownout. The macOS-10.15 environment is deprecated and will be removed on September 30th, 2022. For more details, see actions/runner-images#5583

Azure DevOps build link: https://dev.azure.com/lightgbm-ci/lightgbm-ci/_build/results?buildId=13407&view=logs&j=382e14e0-e893-5ccd-73ac-68042457c8d8

Refer to #5391.

We have #5396 to fix this, but it's blocked by #5390, which is blocked by the need to switch to a new image for building wheels with a more recent version of glibc (#5390 (comment)).

As that error message says, we have until September 30th to get this all done.

I'll prioritize changing that image and start working on it today.

[github-actions]

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