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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❌ 3 New Failures, 2 Cancelled Jobs, 1 Pending

As of commit 44c7409 with merge base 1533e55 ():

NEW FAILURES - The following jobs have failed:

• Lint / lintrunner-mypy (gh)
  >>> Lint for backends/arm/_passes/fuse_concat_pass.py:
• pull / unittest / macos / macos-job (gh)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_mv3_model
• pull / unittest-arm-backend-with-no-deps (test_pytest_ops_tosa) / linux-job (gh)
  RuntimeError: Command docker exec -t 232fc3bcbc10e6381990cab7169a40ca097993fc8a0e3fa47356f4f31da8ece6 /exec failed with exit code 1

CANCELLED JOBS - The following jobs were cancelled. Please retry:

• pull / unittest / windows / windows-job (gh)
• pull / unittest-editable / windows / windows-job (gh)

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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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