Add FuseConcatPass to eliminate redundant concat ops (#18827)

[ryan-monroe]

Summary:

Concat (torch.cat) in the Gen2 Executorch ARM/Ethos-U stack is lowered to
TOSA CONCAT, which Vela then converts to N x MemoryCopy operations — real
DMA data movement on the NPU. This pass eliminates concat operations that
can be proven unnecessary at the FX graph level, preventing Vela from
generating MemoryCopy ops entirely.

Inspired by Espresso's concat elimination techniques
(bolt/nn/espresso/transforms/remove_nops.py), three patterns are handled:

1. Single-input concat: cat([x]) is a no-op, replaced with x.
2. Concat-then-slice: if every consumer of cat([a, b, ...]) is a
   slice_copy that extracts exactly one original input, bypass both.
3. Slice-then-concat: if contiguous slices of the same tensor are
   concatenated back, the result is the original tensor.

Differential Revision: D97667069

[pytorch-bot]

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[meta-codesync]

@ryan-monroe has exported this pull request. If you are a Meta employee, you can view the originating Diff in D97667069.

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[ryan-monroe]

@pytorchbot rerun -f

[pytorch-bot]

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[pytorch-bot]

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[Ninja91]

@gggekov Here's another optimization pass we plan to add. Please comment if this will impact downstream passes/regor transforms

[gggekov]

I recommend testing the passes using EthosU85PipelineINT or EthosU55PipelineINT in order to know if the pass works with the current version of Vela.

[gggekov]

Overall, for me it makes sense for the optimization of some CONCATs to be done in ExecuTorch rather than Vela.

[gggekov]

Helo @ryan-monroe ,
Thanks for the patch, great initiative to reduce the number of memcpy from CONCATs!

I see in the unit tests, you only test with the executorch.backends.arm.test.tester.test_pipeline.PassPipeline. Given that the end metric you care for is number of CONCATs in the TOSA flatbuffer, wouldn't it make sense to test instead either with the EthosU85PipelineINT / EthosU55PipelineINT or TosaPipelineINT as these pipelines will generate a TOSA fb that is run on device? For counting the number of CONCAT ops in the TOSA fb, you can use the count_tosa_ops in backends/arm/test/tester/test_pipeline.py. See example how to do that here - https://github.com/pytorch/executorch/blob/main/backends/arm/test/misc/test_transpose_counts.py#L537

[oscarandersson8218]

Don't you need to normalize both start and end as these can be negative?