Make python TensorOption signatures consistent with JIT schemas (#82241)
Fixes #81774
`TensorOptions` arguments in the JIT schema are optional, but in the Python API these were being translated to non-optional but with a default value. This change makes the arguments accept `None` for consistency with the JIT schema. However, it also means that `dtype=c10::nullopt` was previously completely untested so this also fixes several related bugs.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/82241
Approved by: https://github.com/ngimel
