Modify the implementation of the forward and inverse methods for the rational quadratic spline to improve speed when using TPUs.

[copybara-service]

Modify the implementation of the forward and inverse methods for the rational quadratic spline to improve speed when using TPUs.

The main change is to avoid fancy indexing of the spline parameters. Instead we reduce with a one-hot mask the bin parameters of the correct splines prior to the spline and logdet computation.

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