[pipelining] Add tests for tracing frontend

[kwen2501]

Stack from ghstack (oldest at bottom):

• -> [pipelining] Add tests for tracing frontend #125449
• [pipelining] Add tracing frontend #125448
• [pipelining] Add microbatch split and merge utils #125273

cc @mrshenli @pritamdamania87 @zhaojuanmao @satgera @rohan-varma @gqchen @aazzolini @osalpekar @jiayisuse @H-Huang @awgu @penguinwu @fegin @XilunWu @wanchaol @fduwjj @wz337 @tianyu-l @wconstab @yf225 @chauhang @d4l3k

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[wconstab]

lgtm.

the unflatten test reminds me, we need a doc for unflattener, and in the doc we should be pushing ourselves to tighten up what the actual contract with unflattener is and what the semantics of the returned module tree is.

[kwen2501]

Good idea.
Will doc it at the pipeline API level (unflattener is private).

[kwen2501]

@pytorchbot merge

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