Using rational function approximation for tanh() and sigmoid.

CuDNNGRU and GRU don't use the same weight format

This isn't necessary since valid exponents can't flip the sign bit

See https://github.com/mozilla/LPCNet/commit/bffdcee95#commitcomment-46372726

When implementing using SSSE3 or AVX2, our dot products can saturate
if two adjacent weights sum to more than 127.

Not sure why CuDNNGRU doesn't get used by default, but we need
to explicitly use it to get things to run fast.

was increased too much in 713d53e8a

Reciprocal approximation could cause the sigmoid output to be
greater than 1.0.

Would cause unused pitch embedding vectors to be used

No change otherwise