Fix-ups to the RegularStepGradientDescentOptimizerTest
Fix-ups to the RegularStepGradientDescentOptimizerTest

- Removed Trace structure (not really needed to test Sampler)

- Changed to a three-phase optimization where the step size is
  increased in each phase

- Loosened convergence criteria (otherwise would need to run many more
  thousands of iterations)

chunshen1987/DistributionSampling,chunshen1987/DistributionSampling,chunshen1987/DistributionSampling,chunshen1987/DistributionSampling,chunshen1987/DistributionSampling,chunshen1987/DistributionSampling