Replaces switch/case with unordered_map for input shape inference for future scalability.

Summary: Replace switch/case with a map, and extract repetitive code for future scalability (easier development when adding new operators to the shape inference engine).

Reviewed By: JasonHanwen

Differential Revision: D27301983

fbshipit-source-id: 04b53836960c4f8a3855593660a869bca4b9430a

Author: cr-fb