[Shape Inference] Fix InferFC
Summary: Sometimes first dim of X in FC is BATCH_OF_FEATURE_MAX instead of BATCH. This caused an issue in f207899183 (when first dim of X is 64 but is set to 1 in inferFC). Change the check from `!= BATCH` to `== UNKNOWN`
Test Plan: unit test
Reviewed By: yinghai
Differential Revision: D22784691
fbshipit-source-id: eb66ba361d6fe75672b13edbac2fbd269a7e7a00
