Adds float64 support for avg pool and its gradient.

Eigen NumTraits is modified to directly use std::numeric_limits, which resolves a broken test caused by inconsistency between the host and devices values of Eigen::NumTraits<double>::highest(). This returns +inf on device, due to third_party/eigen3/Eigen/src/Core/util/Meta.h, and __DBL_MAX__ (1.7976931348623157e+308) on host, making the behavior for doubles (on device) inconsistent with both the behavior of floats Eigen::NumTraits<float>::highest() and the behavior of std::numeric_limits<double>::max()

PiperOrigin-RevId: 189731521