[python-package] fix mypy error about cpu_count() methods

[jameslamb]

Contributes to #3756.
Contributes to #3867.

Fixes the following mypy errors.

python-package/lightgbm/sklearn.py:696: error: Incompatible return value type (got "Union[int, Any, None]", expected "int")  [return-value]

lightgbm tries a few different approaches to determine the number of phyiscal/logical CPUs based on conditional imports. Adding type hints explicitly telling mypy that those methods will return integers resolves this issue.

In this case, mypy caught a legitimate possible source of bugs 🎉

If joblib.cpu_count can't be imported but psutil.cpu_count can, psutil.cpu_count() is used to determine the number of available CPUs.

According to the docs at https://psutil.readthedocs.io/en/latest/#psutil.cpu_count, that function returns None if it can't determine the number of CPUs. That's problematic because the result of that function is used unconditionally as if it was an integer:

LightGBM/python-package/lightgbm/sklearn.py

Line 695 in 8811063

n_jobs = max(_LGBMCpuCount(only_physical_cores=False) + 1 + n_jobs, 1)

None + 1 - 1
# TypeError: unsupported operand type(s) for +: 'NoneType' and 'int'

To deal with that, this PR proposes falling back to a value of 1 if psutil.cpu_count() returns None. That might mean that the user's physical resources are underutilized, but that's better than a runtime error.

The other two methods can't return None?

Right.

joblib.cpu_count() calls loky.cpu_count() (code link). I'm not 100% sure, but looks to me like like loky.cpu_count() cannot return None based on the code paths at https://github.com/joblib/loky/blob/047d80623b7cf2d43500ed56ee0320b3e41c3f81/loky/backend/context.py#L77.

multiprocessing.cpu_count() raises a NotImplementedError if it can't figure out the number of CPUs: https://docs.python.org/3/library/multiprocessing.html#multiprocessing.cpu_count.

[github-actions]

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