Error Handling: refactor GetXlaTensor and related functions to use status types.

[ysiraichi]

This PR refactors error handling for GetXlaTensor() and related functions (e.g. TryGetXlaTensor(), TryGetXlaTensors(), and ReplaceXlaTensor()), by incorporating status types. At this moment, these changes do not translate into better UX for the user. However, this PR lays the groundwork for improving that.

Key Changes

• GetXlaTensor() returns a StatusOr<XLATensorPtr>
• Deprecate TryGetXlaTensor() in favor of GetXlaTensor()
  • Removing it would be difficult, since the current codegen depends on it and there's no clear replacement
  • Replaced other call-sites with appropriate usage of GetXlaTensor()
• Remove TryGetXlaTensors() (note the plural)
  • There was only one usage
  • Easily replaceable by GetXlaTensor()
• Refactor ReplaceXlaTensor(), returning absl::Status
• Replace all GetXlaTensor() calls with GetValueOrThrow(GetXlaTensor()) (same for GetXlaTensors() -- plural)
• Replace all ReplaceXlaTensor() calls with MaybeThrow(ReplaceXlaTensor())

[zhanyong-wan]

Nice!