[BugFix][UMA] Protect target registration (#13624)

[tqchen]

This PR address fixes for UMA target registration.

• Fix the doc issue [Docs][Bug] Image missing for UMA tutorial #13304
• Continues stalled PR [BugFix][UMA] Bugfix of target attr and fixed link in tutorial #12731

Changes:

• Incorporates all proposed fixes from mentioned PR #12731
• Address test case concerns and discussions from PR #12731
• NEW: Already exiting target cannot be created, explicit error on this.
• NEW: Attributes having special/reserved scope cannot be created explicitly.

It also address proper test cases for all the above.

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[tqchen]

cherrypick of #13624