Use ops directly, stop using builder.load to use ops
This PR mainly handles all places where InferenceBuilder is used to
access any op or a specific implementation for an op.
Instead an op is defined, and its proper implementation is picked inside
and the usage will be transparent to the user.
What was done in the PR:
1) Added missing ops (added a py file with fallback mechanism)
2) Added missing fallback implementations for existing ops
3) removed all usages for builder.load and replaced them with ops instead.
4) a small change to softmax_context signature to fit the fallback signature.
