Neptune Logger assumes unix-style filepath for model checkpoint

[seba-eng]

The Neptune Logger can automatically upload model checkpoints if the callback is used. It generates an "expected_model_path" for that. This path has hard-coded a '/' at the end (see here), which can cause problems on Windows systems because the actual path has (or can have) a backslash at this position. A simple fix would be to use os.path.sep insteat of '/'.

cc @justusschock @awaelchli @akihironitta

[seba-eng]

never mind, there is already a pull request (#10500) for this problem

[awaelchli]

Let's use this issue then to link with the PR (which had no bug report attached).
I interpret that you are reporting a bug.