* fix correct indentation in UOp pretty_print for repeated references
When a UOp was referenced multiple times, the walrus operator notation
(e.g., x0:=) was correctly used for the first occurrence, but subsequent
references had misaligned indentation due to an extra space character.
Fix indentation misalignment in pretty_print() when UOps are referenced
multiple times.
* add simple unit tests for UOp repr
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Co-authored-by: chenyu <chenyu@fastmail.com>
