Add `scalarType` to JitType pyi (#81112)
Fixes mypy errors when calling `.type().scalarType()` on an `_C.Value` object.
Reference
https://github.com/pytorch/pytorch/blob/28776c45e393ec7d6731a5613f5d79d309036d3a/torch/csrc/jit/python/python_ir.cpp#L891-L896
Pull Request resolved: https://github.com/pytorch/pytorch/pull/81112
Approved by: https://github.com/BowenBao
