Add FuseConcatPass to eliminate redundant concat ops

[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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