[pipelining] Add manual pipeline stage (#126123)

Add `ManualPipelineStage` under `_PipelineStage.py`

Fix some type hints since `args_recv_info` can contain more than one RecvInfo. Previously the hint was `Tuple[InputInfo]` which meant it is a tuple of size 1. This is different from `List[InputInfo]` which can contain any number of items. I needed to update to `Tuple[InputInfo, ...]` to make the number of items flexible.

Pull Request resolved: #126123
Approved by: https://github.com/kwen2501