Default initialize fixed point Eigen types.

In certain cases, tensors are filled with default values of the type. But, for these fixed point types, these values were uninitialized. Thus, we would have uninitialized memory access bugs, some of which were caught by MSAN.

PiperOrigin-RevId: 344101137
Change-Id: I14555fda74dca3b5f1582da9008901937e3f14e2