Fix type hints: add py.typed marker and resolve mypy errors

[lanpa]

Description

tensorboardX lacked a py.typed marker (PEP 561), causing mypy to skip analysis entirely when importing from the package. This PR adds the marker and cleans up mypy errors in writer.py.

Technical Details

• tensorboardX/py.typed: Empty marker file; tells mypy/pyright the package ships inline types
• MANIFEST.in: Include py.typed in sdist
• tensorboardX/writer.py: Fix Optional handling, string concatenation type errors, and method signature issues surfaced by mypy
• pyproject.toml: Add mypy dev dependency; configure mypy to ignore generated protobuf files

Before (any downstream project importing tensorboardX):

error: Skipping analyzing "tensorboardX": module is installed, but missing library stubs or py.typed marker  [import-untyped]
    from tensorboardX import SummaryWriter

After:

$ mypy check_import.py
Success: no issues found in 1 source file

Verification Results

• uv run mypy tensorboardX — 0 errors
• All 94 existing tests pass

Related Issue

Checklist

• Code follows project style (make lint)
• Tests passed locally (make test)
• Documentation updated if necessary

---

Self-review: I have verified these changes and they are ready for review.

[lanpa]

@copilot How would you confirm #749 is solved by this PR?

[Copilot]

@copilot How would you confirm #749 is solved by this PR?

The fix can be confirmed by reproducing the exact error from issue #749 before and after:

Before this PR (no py.typed marker), running mypy on any file importing tensorboardX gives:

error: Skipping analyzing "tensorboardX": module is installed, but missing library stubs or py.typed marker  [import-untyped]
    from tensorboardX import SummaryWriter

After this PR, the same check passes cleanly:

# check_import.py
from tensorboardX import SummaryWriter

$ mypy check_import.py
Success: no issues found in 1 source file

The key change is the addition of tensorboardX/py.typed — an empty marker file that signals PEP 561 compliance, telling mypy (and other type checkers) that tensorboardX ships its own inline type annotations and should be analyzed rather than skipped. The MANIFEST.in update ensures this file is included in the distributed package, and the pyproject.toml packages list already picks it up via find_packages().