Simplify JAX lowering rules for cumulative sum
Simplify JAX lowering rules for cumulative sum

Rely on XLA decomposition.

# JAX GPU microbenchmarks

285us for cumsum over 1e8 elements

449us for cumsum over 1e8 elements.

# JAX CPU microbenchmarks:

1.8s vs. 0.7s for 50 iterations over cumsum over 1e7 elements
