* Create helper function for getting threads per
group from a compute shader
Added GetComputeShaderNumThreads() functions to RPIUtils.
By default the function returns 1, 1, 1 in case of errors.
Updated existing code that was looking for 'numthreads' attribute data
with the new GetComputeShaderNumThreads() API.
Signed-off-by: garrieta <garrieta@amazon.com>