With this addition the ComputeCrossEntropy function can be moved
to its own library, because now we can compute it using only the
public API functions from gemma.h
Split common and weights into separate lib
Remove common-inl (does not have to be SIMD code), activations.cc
Centralize switch(Model) to avoid duplication
Move CompressWeightsT to compress_weights.cc
Move LoadWeights to weights.cc
PiperOrigin-RevId: 640869202