GetOutputOp: Refactor and Improve error handling.

[ysiraichi]

This PR improves error handling of LoweringContext::GetOutputOp(), by creating a new LoweringContext::SafeGetOutputOp() function that returns an status-like variable. It also refactors the lowering_context.{h,cpp} C++ files.

Key Changes:

• Created SafeGetOutputOp(), which is an implementation of GetOutputOp() function, but propagates the error status down the line
  • GetOutputOp() calls SafeGetOutputOp()
  • GetOutputOp() users are not immediately affected
• Add missing includes (IWYU ref)
• Remove OutputMap<T> declaration (using PyTorch torch::lazy::OutputMap<T>)