FC Impl is not fitted for int8

Context

Current FCImpl convert every input to the output datatype.

Also the registration system retrieve implementation using output type.

Meaning that int8 implementation will be map to int32 implementation since int8 FC return int32.

Changes required

Update FC Registrar to register based on input AND output.

Update FCImpl.cpp to remove cast on inputs.

Note

@olivierbichler No implementation should cast the inputs!