[Backend] honor unroll factor annotations in SODA backend

[Blaok]

The unroll factor generated by the SODA backend is now derived from loop annotations based on the following rules:

• A >1 IntImm annotate value matched with a "factor" annotate key is an explicit unroll factor
• A For loop without any explicit unroll factor has implicit unroll factor 1
• The unroll factor of a nested loop is the product of all unroll factors underneath
• All stencil stages (i.e., nested loops) should have the same unroll factor
  • Otherwise, the unroll factor of the first stage will be honored and an error message will be printed