Implements MaxPoolGradGrad in tf2xla using bitwise trickery. Further detail covered...

Implements MaxPoolGradGrad in tf2xla using bitwise trickery. Further detail covered by a comment inside pooling_ops.cc.

Retains 32 bits of gradient precision, but can confuse the backprop source for input cells that are equally maximal at 16 bits. We could in principle be accurate up to 31 bits of input, if we were willing to find gradients one bit at a time, or 24 bits of input 8 gradient bits at a time, etc.

PiperOrigin-RevId: 188025278