[XLA:CPU/GPU] Implement the parallel Philox random number generation algorithm.

Implement the RNG elemental ir generator using the Philox algorithm. To ensure
multiple execution of the same RNG hlo instruction rarely produce the same
result, we increment a global variable with the number of random numbers
generated by the RNG hlo each time the hlo is executed and use the value of the
global variable to construct the seed for the RNG algorithm.

Modify the GPU backend to generate a parallel loop to execute the Philox
algorithm. The CPU backend still uses a sequential loop to perform Philox
random number generation, and we will need to enhance the
ParallelTaskAssignment pass to change this.

Remove the old PCG RNG algorithm for the CPU and GPU backends.

PiperOrigin-RevId: 206069733