[Unity][Pass] Reuse prior infra to implement more complete DCE

[ganler]

As a follow-up to #14262, I just noticed that I previously implemented a function called RemoveAllUnused (#14043) that can do function-wise DCE and should be more complete than #14262 as RemoveAllUnused can also remove dead dataflow blocks (added two test-cases to show that).

Sorry, I should have mentioned that earlier to avoid duplication... but still I am putting a patch here in case it is desired.

cc: @spectrometerHBH @tqchen

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[ganler]

At the high level, #14043 is based on a fix-point algorithm to remove unused dataflow statements iteratively. While I think @spectrometerHBH 's impl is more efficient based on the assumption from the dataflow block.

[junrushao]

LGTM

[ganler]

BTW, although this patch uses RemoveAllUnused over @spectrometerHBH 's implementation, it did not remove the prior implementation. Not sure if we should just keep it in case of future usage or just trim it to simplify the code... It would be great if @spectrometerHBH can help decide. :)

[tqchen]

I think we can simply trim it and consolidate the use

[ganler]

For now:

• tvm.relax.transform.DeadCodeElimination is the pass for running DCE over an IRModule.
• tvm.relax.analysis.remove_all_unused is a function for running DCE over a function.

tvm.relax.transform.DeadCodeElimination is implemented based on tvm.relax.analysis.remove_all_unused. I did not sync the function name for the two, as I saw there are other uses for RemoveAllUnused so just want to be conservative first.

[MasterJH5574]

@ganler Thanks for the clarification!