benchmark: add sparse_eye performance benchmark

Add comprehensive benchmark comparing sparse_eye performance with
explicit .coalesce() vs is_coalesced=True parameter. Benchmark tests
various matrix sizes (10 to 50000) across different dtype combinations.

Results show:
- Average speedup: 6.03x
- Speedup range: 3.98x to 14.50x
- Best improvement: 14.50x for large matrices (n=50000, int64/float64)

Speedup increases with matrix size as .coalesce() overhead becomes
more significant for larger index arrays.

Author: aymuos15

torchsparsegradutils/benchmarks/results/sparse_eye_benchmark_results.csv

@@ -0,0 +1,49 @@
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