Unify featurecolumn and column spec

[tonyyang-svail]

@typhoonzero Should feature column and column spec be put in the same struct?

sqlflow/sql/columns/feature_column.go

Lines 38 to 39 in ced5708

	// FIXME(typhoonzero): remove delimiter, dtype shape from feature column
	// get these from column spec claused or by feature derivation.

[typhoonzero]

I'm sure we can put all the information in one struct for each column, and that's a lot of work to do.

[typhoonzero]

I'll do this first before #705 so that it will be more easier to finishi feature derivation.

[tonyyang-svail]

@typhoonzero As my effort in #800 (comment), I find it hard to directly modify the feature column and column spec because various code generators are depending on it.

Since all the code generators are going to be depend on the intermediate representation, We may want to implement the generation of IR first. By implementing functions resolveTrainColumnForIR, resolveAttributeForIR and FeatureDerivationForIR, we can build IR independently. resolveTrainColumnForIR and resolveTrainColumn may contain share some duplicated code, but it is fine.

Then we let all the code generators depend on IR and delete the old resolvers.

.

[typhoonzero]

@tonyyang-svail You are right! I closed #808, and the feature derivation work should also depend on this. I can help finish the IR work first.

[tonyyang-svail]

I am closing this PR because we are no longer attempting to unify feature column and column spec. Please refer to codegen package for IR.